Major update: Start L2/L3 implementation on PC side

Major update: Start L2/L3 implementation on PC side

Using the following changes, it is now possible to receive the PCH and AGCH
messages in the PC-side layer3, as well as trigger RACH sending inside the phone
from the PC:
* merge l1_dedic_mode_data_ind, l1_dedic_mode_data_req and l1_ccch_info_ind into l1_data_ind
* add partial LAPDm implementation (layer2/src/lapdm.c)
* introduce RSLms between LAPDm and L3 (layer2/src/osmocom_rslms.c)
* use new layer1 header field of msgb
* tx_ph_rach_req() and tx_ph_data_req() to send data from PC to target
* implement DEDIC_MODE_DATA_REQ on firmware side