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Enables usage of ARM_ROTPK_LOCATION=regs for FVP board.
Removes hard-coded developer keys. Instead, setting
ARM_ROTPK_LOCATION=devel_* takes keys from default directory.
In case of ROT_KEY specified - generates a new hash and replaces the
original.

Note: Juno board was tested by original feature author and was not tested
for this patch since we don't have access to the private key. Juno
implementation was moved to board-specific file without changing
functionality. It is not known whether byte-swapping is still needed
for this platform.

Change-Id: I0fdbaca0415cdcd78f3a388551c2e478c01ed986
Signed-off-by: Max Shvetsov <maksims.svecovs@arm.com>