add command fitupd to run an update from a FIT image

Command calls update_tftp() analogous to automatic update described
in doc/README.update.

Usage:
fitupd [addr]
        - run update from FIT image at addr
          or from tftp 'updatefile'

Signed-off-by: Andreas Pretzsch <apr@cn-eng.de>
diff --git a/common/Makefile b/common/Makefile
index 124a427..d662468 100644
--- a/common/Makefile
+++ b/common/Makefile
@@ -92,6 +92,7 @@
 COBJS-$(CONFIG_CMD_FDC)$(CONFIG_CMD_FDOS) += cmd_fdc.o
 COBJS-$(CONFIG_OF_LIBFDT) += cmd_fdt.o fdt_support.o
 COBJS-$(CONFIG_CMD_FDOS) += cmd_fdos.o
+COBJS-$(CONFIG_CMD_FITUPD) += cmd_fitupd.o
 COBJS-$(CONFIG_CMD_FLASH) += cmd_flash.o
 ifdef CONFIG_FPGA
 COBJS-$(CONFIG_CMD_FPGA) += cmd_fpga.o