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)

docs/build-flavours/linux-flavour.rst

@@ -22,6 +22,7 @@ Supported machines (default in **bold**):
 
 * **qemux86-64**
 * qemux86
+* qemuarm64-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)