This patch itnroduces all the boilerplate required to build and package
a new libosmo-gprs-sm.so library, which aims to contain the SM layer
implementation.
A minimal implementation is provided together with a unit test,
showcasing Activation of a PDP Context and requesting implicit GPRS
attach to GMM if it is not yet attached.
This commit also adds/improves code in GMM layer implementing the GMMSM
SAP which is used to interact SM<->GMM.
Related: OS#5501
Change-Id: I9b4a9a6364f7799540475e7e1d10ab2310768281
Pau Espin