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Add a new default makefile target to concatenate BL1 and the FIP and
generate armstub8.bin. This way it isn't needed to do it manually.

Documentation updated to reflect the changes.

Change-Id: Id5b5b1b7b9f87767db63fd01180ddfea855a7207
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>