build: add what-jenkins-does target

This target does (pretty much) exactly the same what jenkins
is doing on our build nodes:
 - complete abuild run of our tree with a given payload
 - building all libpayload configs we ship
 - building the cbmem utility

In fact at some point we could tell jenkins to just run this command.

For debugging, pass along V and Q variables so inner make processes
are slightly more noisy on demand.

Change-Id: Ib515170603a151cc3c3b10c743f1468a9875dbdc
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/6797
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Tested-by: build bot (Jenkins)
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>