configs/nanopi_m4: new defconfig

configs/nanopi_m4: new defconfig

A defconfig for the friendlyarm nanopi-m4 was added in
493c3979a4def978a1137cf8543bf0de50d2615e, but then removed
in a24bd8936f57a590f810e14d7828cf1bddce580e 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:
        - readme updated
        - GPT partition layout
        - 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]