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>

 This is the README for crosstool-NG

 Crosstool-NG follows the autoconf dance. So, to get you

 kick-started, just run:

   ./configure --help

 You will find the documentation in the directory 'docs'.

 Here is a quick overview of what you'll find there:

   0 - Table of content

   1 - Introduction

   2 - Installing crosstool-NG

   3 - Configuring a toolchain

   4 - Building the toolchain

   5 - Using the toolchain

   6 - Toolchain types

   7 - Contributing

   8 - Internals

   A - Credits

   B - Known issues

   C - Misc. tutorials

 You can also point your browser at:

   http://ymorin.is-a-geek.org/projects/crosstool

 Aloha!