This commit creates the ath79/mikrotik subtarget in order to support
MikroTik devices based on Qualcomm Atheros MIPS SoCs.
MikroTik devices need a couple of specific features: the split MiNOR
firmware MTD format, which is not used by other devices, and the 4k
sector erase size on SPI NOR storage, which can not be added to the
ath79/generic and ath79/nand subtargets now.
Additionally, the commit moves the two MikroTik devices already in
the generic and nand subtargets to this new one.
Tested on the RB922 board and the wAP AC router.
Signed-off-by: Roger Pueyo Centelles <roger.pueyo@guifi.net>
