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Since those dependencies are phony targets and therefore always out of
date, a normal dependency would cause the .config target to be rebuilt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>