Boot: Update documentation

Update the secure boot user guide with the multiple image
boot and hardware key integration features.

Change-Id: I2cc825ac719c5a16f0af1e66b4fe6b84e7074977
Signed-off-by: Tamas Ban <tamas.ban@arm.com>
Signed-off-by: David Vincze <david.vincze@arm.com>
1 file changed