new default shortcut to config & build a board

The majority of the time that I build things in U-Boot, I want to just
build for the board.  I don't make board config tweaks after selecting the
board.  So add a new pattern rule that allows people to combine two steps
in one go:
	`make foo_config && make` => `make foo`

This shouldn't conflict with any existing make rules as the pattern rule
is used only the rule doesn't already exist.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>