blessed-builds: Allow uploads to 'amber'

currently we only allow uploads to amber-phone however several packages make sense in pureos proper as well:

chatty (not yet uploaded)
libhandy (by hand)
phosh (by hand)
phoc (by hand)

These are currently either uploaded there by hand or not at all, let's fix that by allowing amber as upload target as well and build against amber-proposed-updates in this case.

/cc @jeremiah.foster @arno.bauernoppel if that sounds sensible

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information