aemfvp-a-rme: update boot script

This change reworks the arguments for boot script to make them
mandatory and implements error checking and handling.
The logic to handle the parsing of the strings to detect successfull
boots was simplified.

Signed-off-by: Nuno Lopes <nuno.lopes@arm.com>
Change-Id: I9d4b1ad54a2fe4ec88cc81aa8be710a3d3af6569