Build: Stop cloning mcuboot submodules

Signed-off-by: Anton Komlev <anton.komlev@arm.com>
Change-Id: I2433caba0b820ef349c4f2d4dfe6a9329a38e85e
diff --git a/lib/ext/mcuboot/CMakeLists.txt b/lib/ext/mcuboot/CMakeLists.txt
index a37b1e4..03f888e 100644
--- a/lib/ext/mcuboot/CMakeLists.txt
+++ b/lib/ext/mcuboot/CMakeLists.txt
@@ -25,7 +25,7 @@
         GIT_TAG ${MCUBOOT_VERSION}
         GIT_SHALLOW FALSE
         GIT_PROGRESS TRUE
-        GIT_SUBMODULES ${MCUBOOT_SUBMODULES}
+        GIT_SUBMODULES "${MCUBOOT_SUBMODULES}"
     )
     # ToDo: set GIT_SHALLOW to 'TRUE' when MCUBOOT_VERSION will be set to a tag
     # (instead of a commit hash) with the new release.