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/flash/Makefile.am

@@ -1,11 +1,9 @@
+include $(top_srcdir)/common.mk
+
 SUBDIRS = \
 	nor \
 	nand
 
-AM_CPPFLAGS = \
-	-I$(top_srcdir)/src \
-	-I$(top_builddir)/src
-
 METASOURCES = AUTO
 noinst_LTLIBRARIES = libflash.la
 libflash_la_SOURCES = \