u-boot: update DFU strings after "dynpart"

[Werner Almesberger]

If we started without "mtdparts" set, DFU did not recognize partition names 
even after a subsequent "dynpart". This patch makes "dynpart" update DFU's
partition name table. In revision 4031.

- Werner

----------------------------------- Changes -----------------------------------

uboot-dfu.patch:
- drivers/usb/usbdfu.c (dfu_update_strings): new function to update the USB
  string table after "dynpart"

nand-dynamic_partitions.patch:
- common/cmd_jffs2.c (nand_create_mtd_dynpart): if we have DFU, invoke
  "dfu_update_strings" after "dynpart"