SPI Flash: Add "sf" command

This adds a new command, "sf" which can be used to manipulate SPI
flash. Currently, initialization, reading, writing and erasing is
supported.

Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>
diff --git a/common/Makefile b/common/Makefile
index 9678799..d90b5f2 100644
--- a/common/Makefile
+++ b/common/Makefile
@@ -143,6 +143,7 @@
 COBJS-y += cmd_mac.o
 COBJS-$(CONFIG_CMD_MFSL) += cmd_mfsl.o
 COBJS-$(CONFIG_MP) += cmd_mp.o
+COBJS-$(CONFIG_CMD_SF) += cmd_sf.o
 
 COBJS	:= $(COBJS-y)
 SRCS	:= $(AOBJS:.o=.S) $(COBJS:.o=.c)