Changes

Initial support for >256 BTS

Some users actually want to deploy osmo-bsc connected to more than 256
BTSs. Allow them to do so.

This is about increasing the integer size / maximum value of the
internal "bts->nr" ID usef eg. during VTY configuration.
The ipaccess OML bts_nr is still kept as uint8_t so nothing changes over
the wire. That's because in ipaccess OML, in general each BTS is
bts_nr=0 (bts->bts_nr) under its own ipa connection.

NOTE: This patch doesn't add support to handle >256 BTS over PCUIF,
which still keeps its (struct gsm_pcu_if)->bts_nr as uint8_t.
This is not much of a problem yet though, since the requirement to run
>256BTS is needed in a network where osmo-bts is used, and hence PCUIF
is handled at each BTS.

Related: SYS#7062
Change-Id: I607a68efeb5f4a50cce107d11d3c5126b7d8f81a