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>

 diff -dur ELFkickers.orig/sstrip/Makefile ELFkickers/sstrip/Makefile

 --- ELFkickers.orig/sstrip/Makefile	2001-03-24 12:58:27.000000000 +0100

 +++ ELFkickers/sstrip/Makefile	2007-05-31 21:17:18.000000000 +0200

 @@ -1,7 +1,7 @@

  #  Makefile for sstrip

  sstrip: sstrip.c

 -	gcc -ggdb -Wall -W -o sstrip sstrip.c

 +	$(CC) -Wall -W -o sstrip sstrip.c

  clean:

  	rm -f sstrip