mk: use eval to run mkhelp commands

directly quoting it and running it quoted means
that the shell way try to execute it as a file.

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

mk

@@ -479,7 +479,7 @@ check_makefile()
 
 mkhelp()
 {
-	[ -z "$1" ] || [ -n "$mode" ] || "$1" || $err "mkhelp: !$1"; :
+	[ -z "$1" ] || [ -n "$mode" ] || eval "$1" || $err "mkhelp: !$1"; :
 }
 
 copy_elf()