configs/nanopc_t4: new defconfig

configs/nanopc_t4: new defconfig

A defconfig for the friendlyarm nanopc-t4 was added in
c23895f06e0988319a1959764a402b57bc21ffaf, but then removed
in 8efd4463f78fd68c85b45cc0da347dfb805abb7e because an ARM32
compiler was needed to build ATF, and this was not supported
back then.

Now, since package/arm-gnu-a-toolchain/ has been added, this
defconfig can be re-introduced.

Following new changes are introduced in comparision with the
older changeset:

	- updated readme
	- GPT partition layout is being used
	- update defconfig to build ATF
	- bump u-boot to 2020.07
	- bump linux to 5.8.2

Signed-off-by: Suniel Mahesh <sunil@amarulasolutions.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
[Arnout: fixate U-Boot version; remove blind BR2_TARGET_UBOOT_NEEDS_PYTHON]