refactor(measured-boot): mb algorithm selection

With RSS now introduced, we have 2 Measured Boot backends. Both backends
can be used in the same firmware build with potentially different hash
algorithms, so now there can be more than one hash algorithm in a build.
Therefore the logic for selecting the measured boot hash algorithm needs
to be updated and the coordination of algorithm selection added. This is
done by:

- Adding MBOOT_EL_HASH_ALG for Event Log to define the hash algorithm
to replace TPM_HASH_ALG, removing reference to TPM.

- Adding MBOOT_RSS_HASH_ALG for RSS to define the hash algorithm to
replace TPM_HASH_ALG.

- Coordinating MBOOT_EL_HASH_ALG and MBOOT_RSS_HASH_ALG to define the
Measured Boot configuration macros through defining
TF_MBEDTLS_MBOOT_USE_SHA512 to pull in SHA-512 support if either
backend requires a stronger algorithm than SHA-256.

Signed-off-by: Lauren Wehrmeister <lauren.wehrmeister@arm.com>
Change-Id: I4ddf06ebdc3835beb4d1b6c7bab5a257ffc5c71a
diff --git a/plat/arm/board/fvp/platform.mk b/plat/arm/board/fvp/platform.mk
index 54c5e75..f9053a8 100644
--- a/plat/arm/board/fvp/platform.mk
+++ b/plat/arm/board/fvp/platform.mk
@@ -375,6 +375,10 @@
     $(info Including ${RSS_MEASURED_BOOT_MK})
     include ${RSS_MEASURED_BOOT_MK}
 
+    ifneq (${MBOOT_RSS_HASH_ALG}, sha256)
+        $(eval $(call add_define,TF_MBEDTLS_MBOOT_USE_SHA512))
+    endif
+
     BL1_SOURCES		+=	${MEASURED_BOOT_SOURCES}
     BL2_SOURCES		+=	${MEASURED_BOOT_SOURCES}
 endif