kconfig: remove CONFIG_ from string constants

Having the CONFIG_ prefix in string constants gets in the way of
using a run-time-defined CONFIG_ prefix.

Fix that by using temp growable strings (gstr) in which we printf
the text.

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