fix libosmocore build for NuttX target

this fixes some compilation issues with libosmocore under NuttX,
particularly as some #defines are missing or some header files are
slightly different.
vty: Check with the application before writing the config

For the BSC/NITB application we see that people modify the band
without modifying the ARFCN. This creates an unbootable config.
Using the new hook the BSC/NITB can check if the config is
consistent and prevent the config file being written.

Related: SYS#739
gtp: Add a global region for the gtp library of OpenGGSN

We want to use libosmocore/libosmovty in the GGSN sourcecode
and reserve a global region here.