Build: add cmake property to manage BL2_TRAILER_SIZE

Add cmake property to manage the trailer size in the code
and during the images building. If needed, a platform can
change this value in its config file.

Change-Id: Ifeed6ca2bf9a15c0f78880902f6cba8b0f9ed602
Signed-off-by: Ludovic Barre <ludovic.barre@foss.st.com>
diff --git a/bl2/ext/mcuboot/mcuboot_default_config.cmake b/bl2/ext/mcuboot/mcuboot_default_config.cmake
index 6f15591..593971a 100644
--- a/bl2/ext/mcuboot/mcuboot_default_config.cmake
+++ b/bl2/ext/mcuboot/mcuboot_default_config.cmake
@@ -16,6 +16,7 @@
 set(MCUBOOT_HW_KEY                      ON          CACHE BOOL      "Whether to embed the entire public key in the image metadata instead of the hash only")
 set(MCUBOOT_UPGRADE_STRATEGY            "OVERWRITE_ONLY" CACHE STRING "Upgrade strategy for images")
 set(BL2_HEADER_SIZE                     0x400       CACHE STRING    "Header size")
+set(BL2_TRAILER_SIZE                    0x400       CACHE STRING    "Trailer size")
 
 # Specifying a scope of the accepted values of MCUBOOT_UPGRADE_STRATEGY for
 # platforms to choose a specific upgrade strategy for images. These certain