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/CommonConfig.cmake b/CommonConfig.cmake
index 51ddc6a..8418922 100644
--- a/CommonConfig.cmake
+++ b/CommonConfig.cmake
@@ -32,6 +32,7 @@
 set(BUILD_CMSIS_DRIVERS Off)
 set(BUILD_UART_STDOUT Off)
 set(BUILD_FLASH Off)
+set(BUILD_BOOT_SEED Off)
 if(NOT DEFINED PLATFORM_CMAKE_FILE)
 	message (FATAL_ERROR "Platform specific CMake is not defined. Please set PLATFORM_CMAKE_FILE.")
 elseif(NOT EXISTS ${PLATFORM_CMAKE_FILE})