gprs: Move TLV parser functions to gprs_utils.c and rename them

Currently the parser functions for single information elements are
defined within gprs_gb_parse.c and not exported explicitely. In
addition they are named like libosmocore's TLV parser functions and
do not have a proper name prefix. Since it is planned to use them for
other protocols, they need to be globally accessible.

This patch moves them to gprs_utils.c and renames them.

The new names are:
    lv_shift             -> gprs_shift_lv
    v_fixed_shift        -> gprs_shift_v_fixed
    lv_shift             -> gprs_shift_lv
    v_fixed_shift        -> gprs_shift_v_fixed

In the long term, these functions should be moved to libosmocore (and
renamed again).

Sponsored-by: On-Waves ehf
diff --git a/openbsc/tests/gprs/gprs_test.ok b/openbsc/tests/gprs/gprs_test.ok
index 39d37c1..56d4b65 100644
--- a/openbsc/tests/gprs/gprs_test.ok
+++ b/openbsc/tests/gprs/gprs_test.ok
@@ -13,4 +13,5 @@
 N(U) = 510, V(UR) = 511 => retransmit
 N(U) = 481, V(UR) = 511 => retransmit
 N(U) = 479, V(UR) = 511 => new
+Test shift functions
 Done.