[api] don't crash when trying to submit to remote instance
[backend] allow to specify external pool repository URLs.

bnc#884270
[backend] ignore frozenlink if a specific srcmd5 is requested
[backend] Add Debian ppc64el architecture to debarches

Debian based distros have different name for PowerPC 64Bit Little Endian
architecture.

Signed-off-by: Dinar Valeev <dvaleev@suse.com>
[backend] write PRODUCT_RELEASE only for SLE12 media style
[backend] export pool repository URL also in productrepositorylist

bnc#884270
[backend] remove dst parameter from update_dst_full()

It is always "$gdst/$packid"...
[backend] simplify fctx_add_binary_to_full to no longer use dst
[backend] detect and fix out of sync bininfos when rebuilding the full tree
[backend] do not fetch imported binaries when getting the old package build result
[api] fix migration numbering
[api] add initial binary tracking code