ipq40xx: add Edgecore OAP-100 support

ipq40xx: add Edgecore OAP-100 support

flashing the unit
* first update to latest edcore FW as per the PDF instructions
* boot the initramfs
  - tftpboot 0x88000000 openwrt-ipq40xx-generic-edgecore_oap100-initramfs-fit-uImage.itb; bootm
* inside the initramfs call the following commiands
  - ubiattach -p /dev/mtd0
  - ubirmvol /dev/ubi0 -n0
  - ubirmvol /dev/ubi0 -n1
  - ubirmvol /dev/ubi0 -n2
* scp the sysupgrade image to the board and call
  - sysupgrade -n openwrt-ipq40xx-generic-edgecore_oap100-squashfs-nand-sysupgrade.bin

Signed-off-by: John Crispin <john@phrozen.org>