Tweak bootloader deb fetching to fix external build

- Because LK source isn't public, we need to fetch it as a binary from
apt in the case that the source is not available.
- Split the fetch step into one fetch for lk and one for
excelsior-bootloader, so that build targets only fetch what they need
(and don't cause issues like having multiple versions of the
excelsior-bootloader package in the output).

Change-Id: I51df316a6616a77851a9dea9a4878f3ac17f066e