SDK: new flash support instructions

From Teltonika Networks Wiki

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.

If you compiled a firmware before, you must execute this command while in the GPL directory before compiling again: 

make target/linux/clean
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