qemu: add support for host-qemu-system

This commit adds support for building the system emulation mode in
host-qemu. To do so, it adds the BR2_PACKAGE_HOST_QEMU_SYSTEM_MODE and
BR2_PACKAGE_HOST_QEMU_LINUX_USER_MODE sub-options, making sure that the
latter is selected if the former is not enabled. This ensures that at
least one is enabled *and* that existing configurations continue to
build the user-land emulation (which was the only one we supported until
now).

The list of architectures supported by the system emulation mode is the
same as the one for the user-space emulation mode (as far as the
existing list is concerned), so we simply drop the comment about this
dependency list being related to the user-space emulation only.

Signed-off-by: Simon Maes <simonn.maes@gmail.com>
[Thomas:
 - Make sure either BR2_PACKAGE_HOST_QEMU_LINUX_USER_MODE or
   BR2_PACKAGE_HOST_QEMU_SYSTEM_MODE get selected.
 - Unconditionally enable FDT support in host-qemu when in system
   mode (so the option BR2_PACKAGE_HOST_QEMU_FDT has been removed)
 - Remove the unneeded BR2_PACKAGE_HOST_QEMU_HAS_EMULS option.
 - Remove the SDL related option, we really on the system to provide
   SDL.
 - Remove the BR2_PACKAGE_HOST_QEMU_DEBUG and
   BR2_PACKAGE_HOST_QEMU_STRIP_BINARY options, since they are not really
   useful.
 - Remove HOST_QEMU_SITE and HOST_QEMU_SOURCE definitions, since they
   are automatically derived from QEMU_SITE and QEMU_SOURCE anyway.
 - Group things more logically in the .mk file.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/qemu/Config.in.host

@@ -1,8 +1,7 @@
 config BR2_PACKAGE_HOST_QEMU
 	bool "host qemu"
-	# So far, we only build the user mode emulation, so this list
-	# of architecture dependencies only takes into account this
-	# emulation mode.
+	select BR2_PACKAGE_HOST_QEMU_LINUX_USER_MODE \
+	       if !BR2_PACKAGE_HOST_QEMU_SYSTEM_MODE
 	depends on BR2_arm       || BR2_armeb       || BR2_aarch64      || \
 		BR2_i386         || BR2_m68k        || BR2_microblazeel || \
 		BR2_microblazebe || BR2_mips        || BR2_mipsel       || \
@@ -12,6 +11,24 @@ config BR2_PACKAGE_HOST_QEMU
 	help
 	  QEMU is a generic and open source machine emulator and virtualizer.
 
-	  This option builds a user emulator for your selected architecture.
+	  This option builds an emulator for your selected architecture.
 
 	  http://www.qemu.org
+
+if BR2_PACKAGE_HOST_QEMU
+
+comment "Emulators selection"
+
+config BR2_PACKAGE_HOST_QEMU_SYSTEM_MODE
+	bool "Enable system emulation"
+	help
+	  Enables the build of the system emulator, which allows to
+	  boot an entire system in Qemu.
+
+config BR2_PACKAGE_HOST_QEMU_LINUX_USER_MODE
+	bool "Enable Linux user-land emulation"
+	help
+	  Enables the build of the user-land emulator, which allows to
+	  run user-space applications.
+
+endif

package/qemu/qemu.mk

@@ -61,9 +61,19 @@ endif
 ifeq ($(HOST_QEMU_ARCH),sh4aeb)
 HOST_QEMU_ARCH = sh4eb
 endif
-HOST_QEMU_TARGETS = $(HOST_QEMU_ARCH)-linux-user
 
-ifeq ($(BR2_PACKAGE_HOST_QEMU),y)
+ifeq ($(BR2_PACKAGE_HOST_QEMU_SYSTEM_MODE),y)
+HOST_QEMU_TARGETS += $(HOST_QEMU_ARCH)-softmmu
+HOST_QEMU_OPTS += --enable-system --enable-fdt
+HOST_QEMU_DEPENDENCIES += host-dtc
+else
+HOST_QEMU_OPTS += --disable-system
+endif
+
+ifeq ($(BR2_PACKAGE_HOST_QEMU_LINUX_USER_MODE),y)
+HOST_QEMU_TARGETS += $(HOST_QEMU_ARCH)-linux-user
+HOST_QEMU_OPTS += --enable-linux-user
+
 HOST_QEMU_HOST_SYSTEM_TYPE = $(shell uname -s)
 ifneq ($(HOST_QEMU_HOST_SYSTEM_TYPE),Linux)
 $(error "qemu-user can only be used on Linux hosts")
@@ -84,12 +94,16 @@ HOST_QEMU_COMPARE_VERSION = $(shell test $(HOST_QEMU_HOST_SYSTEM_VERSION) -ge $(
 # built with kernel headers that are older or the same as the kernel
 # version running on the host machine.
 #
+
 ifeq ($(BR_BUILDING),y)
 ifneq ($(HOST_QEMU_COMPARE_VERSION),OK)
 $(error "Refusing to build qemu-user: target Linux version newer than host's.")
 endif
-endif
-endif
+endif # BR_BUILDING
+
+else # BR2_PACKAGE_HOST_QEMU_LINUX_USER_MODE
+HOST_QEMU_OPTS += --disable-linux-user
+endif # BR2_PACKAGE_HOST_QEMU_LINUX_USER_MODE
 
 define HOST_QEMU_CONFIGURE_CMDS
 	cd $(@D); $(HOST_CONFIGURE_OPTS) ./configure    \
@@ -100,7 +114,8 @@ define HOST_QEMU_CONFIGURE_CMDS
 		--host-cc="$(HOSTCC)"                   \
 		--python=$(HOST_DIR)/usr/bin/python2    \
 		--extra-cflags="$(HOST_CFLAGS)"         \
-		--extra-ldflags="$(HOST_LDFLAGS)"
+		--extra-ldflags="$(HOST_LDFLAGS)"       \
+		$(HOST_QEMU_OPTS)
 endef
 
 define HOST_QEMU_BUILD_CMDS