mk: simplify the main script check

it's still not perfect, but now it's unambiguous.

the previous generic check was written based on the
fact that xbmk's main script used to also be called
via several symlinks, which is no longer the case.

Signed-off-by: Leah Rowe <leah@libreboot.org>

mk

@@ -5,7 +5,7 @@
 
 set -u -e
 
-if [ "./${0##*/}" != "${0}" ] || [ ! -f "mk" ] || [ -L "mk" ]; then
+if [ "$0" != "./mk" ]; then
 	printf "You must run this in the proper work directory.\n" 1>&2
 	exit 1
 fi