build: add common.mk

Rather than specifying common makefile variables move
them all to a common.mk.

Signed-off-by: Spencer Oliver <ntfreak@users.sourceforge.net>

src/target/Makefile.am

@@ -1,3 +1,4 @@
+include $(top_srcdir)/common.mk
 
 if OOCD_TRACE
 OOCD_TRACE_FILES = oocd_trace.c
@@ -5,10 +6,6 @@ else
 OOCD_TRACE_FILES =
 endif
 
-AM_CPPFLAGS = \
-	-I$(top_srcdir)/src \
-	-I$(top_builddir)/src
-
 BIN2C		= $(top_builddir)/src/helper/bin2char$(EXEEXT_FOR_BUILD)
 
 DEBUG_HANDLER	= $(srcdir)/xscale/debug_handler.bin