build(plat/marvell): add descriptions why some checks are required

This change adds just comments why some checks are required. They check
that ENV variables and external repos are correctly set for TF-A builds.

Signed-off-by: Pali Rohár <pali@kernel.org>
Change-Id: I2f8af5061411c0c92d3875917f4d97b60dc2cf10
2 files changed