Move src/*.{c,h} to src/core/

This way we have all libosmocore.so in an own subdir instead of having
lots of files in the parent dir, which also contains subdirs to other
libraries.
This also matches the schema under include/osmocom/.

Change-Id: I6c76fafebdd5e961aed88bbecd2c16bc69d580e2
diff --git a/src/usb/Makefile.am b/src/usb/Makefile.am
index a1426f9..c8e557b 100644
--- a/src/usb/Makefile.am
+++ b/src/usb/Makefile.am
@@ -17,7 +17,7 @@
 	-no-undefined \
 	$(NULL)
 libosmousb_la_LIBADD = \
-	$(top_builddir)/src/libosmocore.la \
+	$(top_builddir)/src/core/libosmocore.la \
 	$(TALLOC_LIBS) \
 	$(LIBUSB_LIBS)