Fix missing targets for m dist, add fetchers for Jenkins

- We didn't have actual targets for the versions of LK and BL2 that are
DRAM-size specific, so m dist would fail to be able to create them.
- Convert the previous bl2 and lk rules to phony targets called
extract-{bl2/lk}, and create targets for the 1G and 2G versions that
depend on those.
- Add checks for if we're in Jenkins, and fetch the bootloader packages
from the apt pool -- like we do on Enterprise

Change-Id: I94257e8549c7865423696b963f200eefd83b96e5
1 file changed