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>
diff --git a/scripts/kconfig/nconf.c b/scripts/kconfig/nconf.c
index 261f926..ce93e87 100644
--- a/scripts/kconfig/nconf.c
+++ b/scripts/kconfig/nconf.c
@@ -7,6 +7,7 @@
  */
 #define _GNU_SOURCE
 #include <string.h>
+#include <stdlib.h>
 
 #include "lkc.h"
 #include "nconf.h"