build: fetch submodules as required

Also document the unusual git feature we employ for 3rdparty

Change-Id: I1d1c986f9d1c4dd8db687d746dbdeb510679141a
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: http://review.coreboot.org/7243
Tested-by: build bot (Jenkins)
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>