kconfig: get CONFIG_ prefix from the environment

Currently, the CONFIG_ prefix is hard-coded in the kconfig frontends
executables. This means that two projects that use kconfig with
different prefixes can not share the same kconfig frontends.

Instead of hard-coding the prefix in the frontends, get it from the
environment, and revert back to hard-coded value if not found.

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