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 using a shell other than bash.

 Courtesy "Martin Guy" <martinwguy@yahoo.it>

 http://sourceware.org/ml/crossgcc/2008-08/msg00037.html

 --- binutils-2.18.orig/ld/genscripts.sh	2007-08-12 20:00:07.000000000 +0100

 +++ binutils-2.18/ld/genscripts.sh	2008-08-25 16:09:54.000000000 +0100

 @@ -398,9 +398,11 @@

      . $em_script

      em_script=$current_script

    }

 -  fragment()

 +  

 +  alias fragment='em_lineno=$LINENO; fragment_em'

 +  fragment_em()

    {

 -    local lineno=$[${BASH_LINENO[0]} + 1]

 +    local lineno=$[$em_lineno + 1]

      echo >> e${EMULATION_NAME}.c "#line $lineno \"$em_script\""

      cat >> e${EMULATION_NAME}.c

    }