Build: Fix MCUboot git tag

To be able to properly fetch a version indicated by
a tag or a commit hash move the 'v' character from the
cmake file to the value of the MCUBOOT_VERSION variable.

Change-Id: Ied87384636805205f13b1e4735e564d63417dffb
Signed-off-by: Tamas Ban <tamas.ban@arm.com>
diff --git a/lib/ext/mcuboot/CMakeLists.txt b/lib/ext/mcuboot/CMakeLists.txt
index 5fcdfd9..4c817cd 100644
--- a/lib/ext/mcuboot/CMakeLists.txt
+++ b/lib/ext/mcuboot/CMakeLists.txt
@@ -16,7 +16,7 @@
 if ("${MCUBOOT_PATH}" STREQUAL "DOWNLOAD")
     FetchContent_Declare(mcuboot
         GIT_REPOSITORY https://github.com/JuulLabs-OSS/mcuboot.git
-        GIT_TAG v${MCUBOOT_VERSION}
+        GIT_TAG ${MCUBOOT_VERSION}
         GIT_SHALLOW TRUE
         GIT_PROGRESS TRUE
         GIT_SUBMODULES ""