Changes

contrib/systemd/osmo-trx-uhd: fix HOME=

With recent changes, osmo-trx-uhd does not run as root anymore. Change
HOME=%h, which evaluates to HOME=/root, to HOME=%S/osmocom which
evaluates to HOME=/var/lib/osmocom. This is the StateDirectory that is
owned by the osmocom user.

Fix for:
  osmo-trx-uhd[4013]: terminate called after throwing an instance of 'boost::filesystem::filesystem_error'
  osmo-trx-uhd[4013]:   what():  boost::filesystem::status: Permission denied: "/root/.config/uhd.conf"
  osmo-trx-uhd[4013]: signal 6 received

Fixes: OS#6495
Change-Id: Ic6f020dd800eedc5e56c7d71e65521ec831d8c64