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/bl2/ext/mcuboot/bl2_main.c b/bl2/ext/mcuboot/bl2_main.c
index 33d2156..08dccfb 100644
--- a/bl2/ext/mcuboot/bl2_main.c
+++ b/bl2/ext/mcuboot/bl2_main.c
@@ -192,12 +192,14 @@
             ;
     }
 
+#ifndef MCUBOOT_USE_UPSTREAM
     rc = boot_nv_security_counter_init();
     if (rc != 0) {
         BOOT_LOG_ERR("Error while initializing the security counter");
         while (1)
             ;
     }
+#endif /* !MCUBOOT_USE_UPSTREAM */
 
     rc = boot_go(&rsp);
     if (rc != 0) {