sgi/secure_boot: copy the prebuilt nor2 flash image

In case of automated execution, it is not possible to manually register
the secure keys into secure variable storage. So copy a prebuilt nor
flash image that is pre-loaded with these keys.

Change-Id: I5fd8607827e236374a998fb5161b843b30f1dbbb
Signed-off-by: Jagadeesh Ujja <jagadeesh.ujja@arm.com>