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This change separates building of flash and UART images, so it is possible
to build only one of these images. Also this change allows make to build
them in parallel.

Target mrvl_flash now builds only flash image and mrvl_uart only UART
image. This change reflects it also in the documentation.
Signed-off-by: Pali Rohár <pali@kernel.org>
Change-Id: Ie9ce4538d52188dd26d99dfeeb5ad171a5b818f3