Makefile: split romstage linking to separate rules

Makefile: split romstage linking to separate rules

After change it is more clear how romstage is linked twice and with
what scripts. Also with the change, it is easier to add some
object of static size that need to be re-compiled for the 2nd link.
One such object could be md5sum of executable.

Change-Id: Ib34d1876071a51345c5c7319a0ed937868817fd1
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/803
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>