pcu_l1_if: move direct phy l1if functions to dedicated header file

The function headers for the direct phy related functions (l1if_...) are
currently directly listed in pcu_l1_if.cpp and osmobts_sock.c. Those
functions are basically the API interface definition between the
platform independed PCU code and the platform dependant direct phy code.
Lets put them into a dedicated header file.

Change-Id: Id54b96a4cd498a9860f3f209498132455e2aa6f8
Related: OS#5198
diff --git a/src/Makefile.am b/src/Makefile.am
index e020ffa..e747d74 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -100,6 +100,7 @@
 	gprs_ms_storage.h \
 	gprs_pcu.h \
 	pcu_l1_if.h \
+	pcu_l1_if_phy.h \
 	pcu_vty.h \
 	pcu_vty_functions.h \
 	mslot_class.h \