docs: explain Measured Boot dependency on Trusted Boot

Change-Id: I04d9439d5967e93896dfdb0f3d7b0aec96c743f9
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>

Makefile

@@ -735,6 +735,9 @@ ifeq ($(CTX_INCLUDE_MTE_REGS),1)
     endif
 endif
 
+# Trusted Boot is a prerequisite for Measured Boot. It provides trust that the
+# code taking the measurements and recording them has not been tampered
+# with. This is referred to as the Root of Trust for Measurement.
 ifeq ($(MEASURED_BOOT),1)
     ifneq (${TRUSTED_BOARD_BOOT},1)
         $(error MEASURED_BOOT requires TRUSTED_BOARD_BOOT=1)

docs/getting_started/build-options.rst

@@ -463,7 +463,10 @@ Common build options
    the build. The default value is 40 in debug builds and 20 in release builds.
 
 -  ``MEASURED_BOOT``: Boolean flag to include support for the Measured Boot
-   feature. If this flag is enabled ``TRUSTED_BOARD_BOOT`` must be set.
+   feature. If this flag is enabled ``TRUSTED_BOARD_BOOT`` must be set as well
+   in order to provide trust that the code taking the measurements and recording
+   them has not been tampered with.
+
    This option defaults to 0 and is an experimental feature in the stage of
    development.