OE has buggy inheritance for machine configs.
meta-arm/qemuarm64-secureboot.conf can not be overwitten by
priority from meta-ts. So the only way I see is to create
a different machine name - tsqemuarm64-secureboot. This
commit also fixes dependencies between tf-a and u-boot. Uses
wic image for final firmware (the same as for other platforms).
And updates documentation.

Signed-off-by: Maxim Uvarov <maxim.uvarov@linaro.org>