Boot: Emphasize the requirement of immutability of root of trust

Add a note to TF-M secure boot documentation which clarifies
that the first stage bootloader and ROTPK must be stored in
an immutable way to accomplish a root of trust anchor.

Change-Id: Ibd3ef9f2e4d176dcfd92fc9a51570fb47b09fc64
Signed-off-by: Tamas Ban <tamas.ban@arm.com>
1 file changed