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>

 Enable building a pure soft-float compiler without the need for a software

 floating point library.

 diff -dur gcc-4.0.4.orig/gcc/config.gcc gcc-4.0.4/gcc/config.gcc

 --- gcc-4.0.4.orig/gcc/config.gcc	2007-02-02 19:12:28.000000000 +0100

 +++ gcc-4.0.4/gcc/config.gcc	2007-02-02 19:12:07.000000000 +0100

 @@ -677,7 +677,7 @@

  		tm_defines="TARGET_BIG_ENDIAN_DEFAULT=1 $tm_defines"

  		;;

  	esac

 -	tmake_file="${tmake_file} arm/t-arm arm/t-linux"

 +	tmake_file="${tmake_file} arm/t-arm arm/t-linux arm/t-arm-elf"

  	extra_parts="crtbegin.o crtbeginS.o crtend.o crtendS.o"

  	gnu_ld=yes

  	;;