Add support for qemuarm/64 machines with EFI

Fixes #667 (closed) Fixes docs#126 (closed)

TODO (do not merge if any pending):

• update docs
• mention new machines in build templates
• upstream edk2 and grub/grub-efi fixes - #766
• upstream the machines - #767 (closed)
• ci
• add ci jobs docs - #765
• drop unsupported machines from everywhere in this repo (qemuarm, qemuarm64, generic...)
• cleanup machine defconfigs - #770 (closed)

flavours/linux/conf-notes.txt

@@ -14,6 +14,8 @@ Supported targets:
 Supported machines (first is the default):
 - qemux86-64
 - qemux86
+- qemuxarm-efi
+- qemuxarm64-efi
 - seco-intel-b68 (SECO SBC-B68)
 - seco-imx8mm-c61-2gb (SECO SBC-C61 2GB DRAM)
 - seco-imx8mm-c61-4gb (SECO SBC-C61 4GB DRAM)