WIP: Add support for building images for Pine A64+

[SunilMohanAdapa]

Signed-off-by: Sunil Mohan Adapa [email protected]

The current state is that images are built successfully. Boot happens for SPL, ATF and u-boot. u-boot is able load the boot script, then kernel, dtb and initrd. However:

• Kernel does not show any messages or show any further progress (such as running systemd or initrd). I assumed that the upstream/Debian kernel does not have enough board support bits to actually boot the board. More work is scheduled to be mered into next kernel releases. See https://linux-sunxi.org/Linux_mainlining_effort .
• The dtb path in /boot is placed by flash-kernel in the wrong path due to a bug in flash-kernel: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=860304. This affects many arm64 boards and can be easily worked around.

[SunilMohanAdapa]

Many basic features such DRAM and Ethernet for Allwinner A64 are available only in Linux 4.15. We should at least wait until Linux 4.15 is available in Debian.