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>

 --- gcc/gcc/config.gcc.uclibc100-sh~	2006-03-06 20:46:56 +0100

 +++ gcc/gcc/config.gcc	2006-03-10 15:02:41 +0100

 @@ -1905,7 +1905,7 @@

  	;;

  sh-*-elf* | sh[12346l]*-*-elf* | sh*-*-kaos* | \

  sh-*-symbianelf* | sh[12346l]*-*-symbianelf* | \

 -  sh-*-linux* | sh[346lbe]*-*-linux* | \

 +  sh*-*-linux* | sh[346lbe]*-*-linux* | \

    sh-*-netbsdelf* | shl*-*-netbsdelf* | sh5-*-netbsd* | sh5l*-*-netbsd* | \

     sh64-*-netbsd* | sh64l*-*-netbsd*)

  	tmake_file="${tmake_file} sh/t-sh sh/t-elf"