trees: exit with error if project undefined

i was originally looser about this, because i also wanted
the trees script to generically run "make" from any
directory, but this behaviour was error-prone and it is
no longer used in the build system.

disable it, in the interest of stability.

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

script/trees

@@ -34,6 +34,9 @@ main()
 	done
 	[ -z "$_f" ] && $err "missing flag (-m/-u/-b/-c/-x/-f/-s/-l/-n)"
 	[ -z "$project" ] && $err "project name not specified"
+
+	[ -f "config/git/$project" ] || $err "project '$project' not defined"
+
 	elfdir="elf/$project"
 	datadir="config/data/$project"
 	cfgsdir="config/$project"