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>

 Don't generate RPATH if we're going to be generating RUNPATH.

 need to ponder what ramifications this has before enabling it

 --- binutils/bfd/elflink.c

 +++ binutils/bfd/elflink.c

 @@ -5382,11 +5382,15 @@

  	  indx = _bfd_elf_strtab_add (elf_hash_table (info)->dynstr, rpath,

  				      TRUE);

 -	  if (indx == (bfd_size_type) -1

 -	      || !_bfd_elf_add_dynamic_entry (info, DT_RPATH, indx))

 +	  if (indx == (bfd_size_type) -1)

  	    return FALSE;

 -	  if  (info->new_dtags)

 +	  if (!info->new_dtags)

 +	    {

 +	      if (!_bfd_elf_add_dynamic_entry (info, DT_RPATH, indx))

 +		return FALSE;

 +	    }

 +	  else

  	    {

  	      _bfd_elf_strtab_addref (elf_hash_table (info)->dynstr, indx);

  	      if (!_bfd_elf_add_dynamic_entry (info, DT_RUNPATH, indx))