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-4.3.2.orig/gcc/config/arm/linux-eabi.h	2009-09-08 23:46:44.000000000 +0200

+++ gcc-4.3.2/gcc/config/arm/linux-eabi.h	2009-09-08 23:46:51.000000000 +0200

@@ -44,7 +44,7 @@

    The ARM10TDMI core is the default for armv5t, so set

    SUBTARGET_CPU_DEFAULT to achieve this.  */

 #undef  SUBTARGET_CPU_DEFAULT

-#define SUBTARGET_CPU_DEFAULT TARGET_CPU_arm10tdmi

+#define SUBTARGET_CPU_DEFAULT TARGET_CPU_arm9tdmi

 /* TARGET_BIG_ENDIAN_DEFAULT is set in

    config.gcc for big endian configurations.  */