variable ./mk location, based on user input

when calling ./mk, or from PATH, an absolute path
is created to ./mk

this will likely be different than mk, but it might
be a different script. perhaps the operator is making
a copy of the mk script temporarily - obviously not
advised, no, but here we have a problem because then
lbmk is still running ./mk everywhere

run arg0 instead, now called "mk"

./mk is still hardcoded in release.sh, because we
do need to make sure we're using the right script
there

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

include/env/get.sh

@@ -32,11 +32,11 @@ single_tree_fetch()
 	fi
 
 	[ -n "$xgcctree" ] && \
-		x_ ./mk -f coreboot "$xgcctree"
+		x_ "$mk" -f coreboot "$xgcctree"
 
 	[ -n "$depend" ] && \
 		for d in $depend ; do
-			x_ ./mk -f $d
+			x_ "$mk" -f $d
 		done
 
 	clone_project