amlogic: switch partition_type to msdos

After commit 8335d050bf24977afefee6612dd347abd555bc53 in pmbootstrap the
partition table now defaults to GPT instead of MBR.  This breaks booting
on Amlogic devices for the following reasons:

 * On set-top boxes: downstream U-Boot most likely does not have GPT
   support
 * On boards where we install mainline U-Boot as BL33 on eMMC/SD: the
   bootROM checks for an `@AML` image header at offset 512 (second LBA),
   which means we are basically stuck on MBR

Some Amlogic boards have U-Boot + the proprietary Amlogic firmware
stages in SPI flash (mainly the Libre Computer boards do), which means
they can use GPT, because the firmware is simply not stored on eMMC/SD.
For that reason I chose not to change the partition table on those.

Signed-off-by: Ferass El Hafidi <funderscore@postmarketos.org>