support/kconfig: use kconfig-provided way of setting the CONFIG_ prefix

It's now been a while since it has been possible to build the kconfig
parser to understand a prefix other than CONFIG_, and even no prefix
at all, by setting the CONFIG_ macro (#define) at biuld time.

Just use that, insted of patching, it will make it easier for us in the
future.

Our patches have been refreshed at the same time.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>