Handle Immediate assignment reject

When RACH is received, PCU will generate the Immediate assignment reject
message if no resources are present. The encoding is done based on section
9.1.20 of 44.018 version 11.7.0 Release 11. This patch also includes the
test case to validate the generated Immediate assignment reject message.

This patch is integration tested on Osmo-trx setup with Ettus B210 board
and LG F70 MS with some simulation code changes in Osmo-pcu.

Change-Id: I3d33e2b9746fa4f338fad0e6b63b1c5f07de6f9b
8 files changed