scripts/config: allow alternate prefix to config option symbol

While the Linux kernel uses 'CONFIG_' as a prefix to the config options
symbols, many projects that use kconfig may use different prefixes, or
even none at all.

If the CONFIG_ environment variable is set, use it as the prefix (empty
is a valid prefix). Otherwise, use the default prefix 'CONFIG_'.

This matches the support for alternate prefixes in scripts/kconfig/lkc.h,
which uses the same logic (albeit with a C define instead of an environment
variable).

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