core/pkg-kconfig: allow dependencies before configurators

Some users of kconfig need some packages to be built before their
kconfig infra be used.

For example, the linux kernel, starting with 4.16, needs flex and bison
to generate the parser code. Furthermore, starting with 4.18, it will
also need the cross-compiler before parsing the kconfig stuff, because
that calls the compiler to check its features.

Currently, this is broken, even the flex/bison ones, even though they
are listed, because there is no way to define dependencie that are
guaranteed before the (visual) configurators. For example:

    $ make distclean
    $ make menuconfig
      --> enable the linux kernel, choose a defconfig, save, exit
    $ make linux-menuconfig
    [...]
      HOSTCC  scripts/basic/fixdep
      HOSTCC  scripts/kconfig/conf.o
      YACC    scripts/kconfig/zconf.tab.c
    /bin/sh: bison: command not found
      LEX     scripts/kconfig/zconf.lex.c
    scripts/Makefile.lib:196: recipe for target 'scripts/kconfig/zconf.tab.c' failed
    make[3]: *** [scripts/kconfig/zconf.tab.c] Error 127
    make[3]: *** Waiting for unfinished jobs....
    /bin/sh: flex: command not found
    scripts/Makefile.lib:188: recipe for target 'scripts/kconfig/zconf.lex.c' failed
    make[3]: *** [scripts/kconfig/zconf.lex.c] Error 127
    Makefile:528: recipe for target 'rpc_defconfig' failed
    make[2]: *** [rpc_defconfig] Error 2
    linux/linux.mk:511: recipe for target '/home/ymorin/dev/buildroot/buildroot/output/build/linux-4.17.11/.config' failed
    make[1]: *** [/home/ymorin/dev/buildroot/buildroot/output/build/linux-4.17.11/.config] Error 2
    Makefile:79: recipe for target '_all' failed
    make: *** [_all] Error 2

So, we introduce a new type of dependencies for kconfig-based packages,
that are guaranteed to be built and installed before the (visual)
configurators are called.

Since those dependencies are phony targets and therefore always out of
date, a normal dependency would cause the .config target to be rebuilt
on each invocation of make. So we use an order-only pre-requisite, like
is done for the patch dependency.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Jan Kundrát <jan.kundrat@cesnet.cz>
Tested-by: Jan Kundrát <jan.kundrat@cesnet.cz>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

docs/manual/adding-packages-kconfig.txt

@@ -81,3 +81,7 @@ be set to suit the needs of the package under consideration:
   be well suited for all packages that use the standard kconfig
   infrastructure as inherited from the Linux kernel; some packages use
   a derivative of kconfig that use a different location.
+
+* +FOO_KCONFIG_DEPENDENCIES+: the list of packages (most probably, host
+  packages) that need to be built before this package's kconfig is
+  interpreted. Seldom used. By default, empty.

package/pkg-kconfig.mk

@@ -41,6 +41,10 @@ endef
 
 define inner-kconfig-package
 
+# Register the kconfig dependencies as regular dependencies, so that
+# they are also accounted for in the generated graphs.
+$(2)_DEPENDENCIES += $$($(2)_KCONFIG_DEPENDENCIES)
+
 # Call the generic package infrastructure to generate the necessary
 # make targets.
 # Note: this must be done _before_ attempting to use $$($(2)_DIR) in a
@@ -121,6 +125,11 @@ $$($(2)_DIR)/$$($(2)_KCONFIG_DOTCONFIG): $$($(2)_KCONFIG_FILE) $$($(2)_KCONFIG_F
 # it explicitly. It doesn't hurt to always have it though.
 $$($(2)_DIR)/$$($(2)_KCONFIG_DOTCONFIG): | $(1)-patch
 
+# Some packages may need additional tools to be present by the time their
+# kconfig structure is parsed (e.g. the linux kernel may need to call to
+# the compiler to test its features).
+$$($(2)_DIR)/$$($(2)_KCONFIG_DOTCONFIG): | $$($(2)_KCONFIG_DEPENDENCIES)
+
 # In order to get a usable, consistent configuration, some fixup may be needed.
 # The exact rules are specified by the package .mk file.
 define $(2)_FIXUP_DOT_CONFIG