Prepare for better device categorization by moving everything to testing
subdir first.
[skip-ci]: chicken-egg problem: passing pmaports CI depends on pmbootstrap MR
depends on this MR
Related: postmarketos#16
This updates the uuu flasher script to conform to what Purism uses, and
changes the package to use the new M4 + uboot image that is generated by
the librem5 u-boot package.
It also changes the kernel/uboot to the 'generic' librem5 kernel/uboot.
This simplifies the uboot script and updates it to use correct address
offsets for kernel, ramdisk and fdt. Appending the dtb to the kernel is
no longer necessary. This also changes the dtb to the one used on
PureOS.
