Platform: Add boot seed support

Add a getter function to platform code, which makes possible
to get the boot seed value. Boot seed is a mandatory claim
in the initial attestation token.

Change-Id: Ifd547c541a51093f0766b57ba9e9cdd124cd840d
Signed-off-by: Tamas Ban <tamas.ban@arm.com>
diff --git a/platform/ext/musca_a.cmake b/platform/ext/musca_a.cmake
index ccd5073..985d8b9 100755
--- a/platform/ext/musca_a.cmake
+++ b/platform/ext/musca_a.cmake
@@ -178,3 +178,9 @@
 if (MCUBOOT_NO_SWAP)
 	message (FATAL_ERROR "MCUBOOT_NO_SWAP configuration is not supported on " ${TARGET_PLATFORM})
 endif()
+
+if (NOT DEFINED BUILD_BOOT_SEED)
+  message(FATAL_ERROR "Configuration variable BUILD_BOOT_SEED (true|false) is undefined!")
+elseif(BUILD_BOOT_SEED)
+  list(APPEND ALL_SRC_C "${PLATFORM_DIR}/target/musca_a/dummy_boot_seed.c")
+endif()