xtensa-build-zephyr: add option to stop after cloning Zephyr

This is needed by CI to clone only once and then take control of the
iteration over platform builds. CI already does all that but by
copy/paste/diverge of this clone() function.

Once cloning Zephyr in CI code is de-duplicated and uses this new
clone-only feature instead, it will be possible to submit any Zephyr
commit to SOF testing.  In other words it will make failed attempt
https://github.com/thesofproject/sof/pull/4728 possible

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

scripts/xtensa-build-zephyr.sh

@@ -36,14 +36,15 @@ print_usage()
 Re-configures and re-builds SOF with Zephyr using a pre-installed Zephyr toolchain and
 the _defconfig file for that platform.
 
-usage: $0 [options] platform(s) [ -- cmake arguments ]
+usage: $0 [options] [ platform(s) ] [ -- cmake arguments ]
 
        -a Build all platforms.
        -j n Set number of make build jobs for rimage. Jobs=#cores by default.
            Ignored by "west build".
        -k Path to a non-default rimage signing key.
        -c recursively clones Zephyr inside sof before building.
-          Incompatible with -p.
+          Incompatible with -p. To stop after cloning Zephyr, don't
+          pass any platform or cmake argument.
        -p Existing Zephyr project directory. Incompatible with -c.  If
           zephyr-project/modules/audio/sof is missing then a
           symbolic link pointing to ${SOF_TOP} will automatically be
@@ -282,14 +283,11 @@ parse_args()
 	# Check some target platform(s) have been passed in one way or
 	# the other
 	if [ "${#PLATFORMS[@]}" -eq 0 ]; then
-		echo "Error: No platforms specified. Supported are: " \
-		     "${SUPPORTED_PLATFORMS[*]}"
-		print_usage
-		exit 1
-	fi
-
+		printf 'No platform build requested\n'
+	else
 		printf 'Building platforms:'
 		printf ' %s' "${PLATFORMS[@]}"; printf '\n'
+	fi
 
 	CMAKE_ARGS=("$@")
 	# For debugging quoting and whitespace
@@ -318,7 +316,7 @@ main()
 		)
 	fi
 
-	build_all
+	test "${#PLATFORMS[@]}" -eq 0 || build_all
 }
 
 main "$@"