kconfig: add a function to get the CONFIG_ prefix

Currently, we get the CONFIG_ prefix via the CONFIG_ macro, which means
the CONFIG_ prefix is hard-coded at compile time. This goes against
having a run-time defined CONFIG_ prefix.

Add a function that returns the CONFIG_ prefix to use (but keep the
current hard-coded behavior, to be changed in a later patch).

To avoid touching all the code that uses the CONFIG_ macro, we just
undef it, and define it to be a call to the function.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Michal Marek <mmarek@suse.cz>