configure: add possibility to set arbitrary variable in check_for

If check_for is able to find the required prog/inc/lib, allow it to
set an arbitrary variable to 'y'. This variable is then pushed down
to the kconfig definition.

For example:
has_or_abort prog=foobar kconfig=has_foobar

If foobar is available, it yields a kconfig variable defaulting to y:
config CONFIGURE_has_foobar
bool
default y

If foobar is missing, it yields a kconfig variable defaulting to n:
config CONFIGURE_has_foobar
bool

Thus it is possible to depends on that variabel to show/hide options:
config SOME_FEATURE
bool
prompt "Some feature"
depends on CONFIGURE_has_foobar

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 # powerpc specific configuration file

 config ARCH_POWERPC_SPE

     bool

     prompt "Enable SPE support"

     default n

     help

       Add support for the Signal Processing Engine.  This will set up

       the toolchain so that it supports the SPE ABI extensions. This

       mainly targets Freescale e500 processors.

       Setting this option will append "spe" to the end of your target

       tuple name (e.g., powerpc-e500v2-linux-gnuspe) so that the gcc

       configure/build system will know to include SPE ABI support. It

       will also automatically add "-mabi=spe -mspe" to your TARGET_CFLAGS,

       and "--enable-e500_double" to your CC_EXTRA_CONFIG_ARRAY, so you

       do not need to explicitly add them.