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