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>

 --- binutils/ld/emulparams/elf_i386.sh

 +++ binutils/ld/emulparams/elf_i386.sh

 @@ -13,3 +13,13 @@

  NO_SMALL_DATA=yes

  SEPARATE_GOTPLT=12

  IREL_IN_PLT=

 +

 +# In Gentoo, we install 32bit libs into /lib32 in an

 +# ABI setup with amd64/x86

 +case "$target" in

 +  x86_64*-linux*)

 +    case "$EMULATION_NAME" in

 +      *i386*) LIBPATH_SUFFIX=32  ;;

 +    esac

 +  ;;

 +esac