tools/mkenvimage.c: fix basename(3) usage

tools/mkenvimage.c: fix basename(3) usage

Use the POSIX variant of basename due to BSD systems (e.g. OS X) do not provide
GNU version of basename(3). It is save to use the POSIX variant here cause we do
never use argv[0] later on which may be modified by the basename(3) POSIX
variant.
On systems providing GNU variant the GNU variant should be used since string.h
is included before libgen.h. Therefore let the _GNU_SOURCE as is.

This patch fixes following warning (on OS X):
---8<---
mkenvimage.c: In function ‘main’:
mkenvimage.c:105: warning: implicit declaration of function ‘basename’
mkenvimage.c:105: warning: assignment makes pointer from integer without a cast
--->8---

Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
cc: Keith Mok <ek9852@gmail.com>