kernel: fix include of myloader.o since kernel 5.4

kernel: fix include of myloader.o since kernel 5.4

Between 4.19 and 5.4, mtd parsers have been moved to "parsers"
subdirectory. Based on this, the selection of myloader.o in OpenWrt
was also moved to that subdirectory, while the Kconfig and our local
myloader.c file remained in /drivers/mtd.

This resulted in linking errors like the following (on ath25@5.4):

make[8]: *** No rule to make target 'drivers/mtd/parsers/myloader.o', ...
   needed by 'drivers/mtd/parsers/built-in.a'.  Stop.
make[7]: *** [scripts/Makefile.build:500: drivers/mtd/parsers] Error 2
make[6]: *** [scripts/Makefile.build:500: drivers/mtd] Error 2

Since myloader.c is not too big, this patch moves it to the kernel patches,
allowing to adjust the path for kernel 5.4 and keeping Makefiles and
file paths better in sync.

Other patches have been refreshed accordingly.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>