am335x_evm: Enable DFU for NAND and MMC, provide example alt_infos

- Add CONFIG_DFU_NAND, CONFIG_DFU_MMC
- Set dfu_alt_info_nand, dfu_alt_info_emmc and dfu_alt_info_mmc to show
  working examples for those cases.
- Increase CONFIG_SYS_MAXARGS due to hush parsing bugs that would
  otherwise disallow 'setenv dfu_alt_info ${dfu_alt_info_nand}'.
- Enable CONFIG_FAT_WRITE to allow updating on MMC

Signed-off-by: Pantelis Antoniou <panto@antoniou-consulting.com>
Signed-off-by: Tom Rini <trini@ti.com>