qemu: allow to build host variant statically

Compiling Qemu statically allows to use it to chroot into target/. It is a
nice feature, so add an option for it.

Signed-off-by: Jérôme Pouiller <jezz@sysmic.org>
[Thomas: slightly tweak Config.in help text.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/qemu/Config.in.host

@@ -17,6 +17,34 @@ config BR2_PACKAGE_HOST_QEMU
 
 if BR2_PACKAGE_HOST_QEMU
 
+config BR2_PACKAGE_HOST_QEMU_STATIC
+	bool "Build statically"
+	help
+	  Build qemu as a statically linked executable. Use this
+	  option if you want to 'chroot' in your target directory. The
+	  following steps detail how to achieve this with an ARM
+	  target:
+
+	    1. Copy the qemu binary in your target directory:
+
+	       cp host/usr/bin/qemu-arm target/usr/bin
+
+	    2. Make sure binfmt_misc is mounted on /proc/sys/fs/binfmt_misc:
+
+	       mount -t binfmt_misc none /proc/sys/fs/binfmt_misc
+
+	    3. Register /usr/bin/qemu-arm is interpreter for arm
+	       binaries. See linux/Documentation/binfmt_misc.txt for
+	       more information:
+
+	       MAGIC='\x7f\x45\x4c\x46\x01\x01\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\x28\x00'
+	       MASK='\xff\xff\xff\xff\xff\xff\xff\x00\xff\xff\xff\xff\xff\xff\xff\xff\xfe\xff\xff\xff'
+	       echo ":qemu-arm:M:0:$MAGIC:$MASK:/usr/bin/qemu-arm:OC" > /proc/sys/fs/binfmt_misc/register
+
+	    4. Chroot into your target:
+
+	       chroot target /bin/sh
+
 comment "Emulators selection"
 
 config BR2_PACKAGE_HOST_QEMU_SYSTEM_MODE

package/qemu/qemu.mk

@@ -120,6 +120,10 @@ HOST_QEMU_OPTS += --enable-vde
 HOST_QEMU_DEPENDENCIES += host-vde2
 endif
 
+ifeq ($(BR2_PACKAGE_HOST_QEMU_STATIC),y)
+HOST_QEMU_OPTS += --static
+endif
+
 # Override CPP, as it expects to be able to call it like it'd
 # call the compiler.
 define HOST_QEMU_CONFIGURE_CMDS