Allow for user CFLAGS

Ideally, a user could set CFLAGS on the command line to contain
compilation options and include and library paths (or maybe set
those last two with CPPFLAGS and LDFLAGS). Unfortunately, setting
CFLAGS on the command line currently short-circuits essential
logic in the makefile and breaks the build. After a first failed
attempt it appears that a monolithic overhaul or "flag day" to
fix this is a substantial undertaking, so for now just provide a
workaround in the form of a USERCFLAGS variable.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

Makefile

@@ -33,6 +33,8 @@ SH		:= bash
 MAKE		:= make
 OBJCOPY		:= $(CROSS_COMPILE)objcopy
 
+CFLAGS		+= $(USERCFLAGS)
+
 #
 # Fetch ARCH from the uname if not yet set
 #