fix(sptool): fix concurrency issue for SP packages

Add dependency between rules to generate SP packages and their dtb files
to ensure the dtb files are built before the sptool attempts to generate
the SP package.

Change-Id: I071806f4aa09f39132e3e1990c91d71dc9acd728
Signed-off-by: Daniel Boulby <daniel.boulby@arm.com>