Makefile: let plain 'make' be like 'make all' again

That behaviour was broken by c96724af1baaa926cbcc8d8e5b19fd5ba339d962
(Makefile: add short help target). Also, update the helptext accordingly.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>