Build: Enable building TF-M with original MCUBoot

Enable building TF-M with the original MCUBoot (alongside the forked one
in the TF-M repository). Before building with the upstream MCUBoot repo,
it must be cloned into the TF-M base folder (into which the TF-M was
cloned previously) and the -DMCUBOOT_REPO=UPSTREAM option must be added
to the command line at the CMake configuration step.
The MCUBOOT_REPO option determines the repository from which it will use
the MCUBoot. By default it will use it from the TF-M repository.

Add description of how to build TF-M with upstream MCUBoot to the
documentation.

Change-Id: I2cfa55039943a9ac919156570120367d9603a816
Signed-off-by: David Vincze <david.vincze@arm.com>
diff --git a/CommonConfig.cmake b/CommonConfig.cmake
index cf711eb..63b2a15 100644
--- a/CommonConfig.cmake
+++ b/CommonConfig.cmake
@@ -426,7 +426,7 @@
 validate_cache_value(ATTEST_BOOT_INTERFACE)
 
 if (NOT DEFINED BOOT_DATA_AVAILABLE)
-	if (BL2)
+	if (BL2 AND (NOT MCUBOOT_REPO STREQUAL "UPSTREAM"))
 		set(BOOT_DATA_AVAILABLE ON)
 	else()
 		set(BOOT_DATA_AVAILABLE OFF)