SDK: new flash support instructions

From Teltonika Networks Wiki

RUT230, RUT240, RUT850, RUT900, RUT950 and RUT955 devices will be manufactured with a new type of flash memory starting from the second half of 2019 (click on one of the previous links for more details). Firmwares compiled on older than RUT2XX_R_00.01.07, RUT850_R_00.01.03.4, RUT9XX_R_00.06.02 SDK versions will not work with these devices, because they lack the new flash support.

To solve this issue, move the 473-mtd-m25p80-add-support-for-XTX-flash-and-undefined-flash.patch file to the target /target/linux/ar71xx/patches-3.18/ directory and re-compile the firmware.