libpayload: Provide dummy getenv()

Change-Id: I419fcb16e0b10dee9195072e0e6befa6c9e61a69
Signed-off-by: Patrick Georgi <patrick.georgi@secunet.com>
Reviewed-on: http://review.coreboot.org/105
Tested-by: build bot (Jenkins)
Reviewed-by: Frank Vibrans III <frank.vibrans@amd.com>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

payloads/libpayload/include/stdlib.h

@@ -143,5 +143,5 @@ void exit(int status) __attribute__ ((noreturn));
 /** @} */
 
 void qsort(void *aa, size_t n, size_t es, int (*cmp)(const void *, const void *));
-
+char *getenv(const char*);
 #endif