Fix 'date' so that it does not cause regeneration for unbundled builds

Currently a 'tapas' build will regenerate each time with:

    $(shell date +%m%d%y_%H%M%S) was changed, regenerating...

Change-Id: I2eeda1e9dd51a56bf0783f427b4fcf5a1f489254
Merged-In: I2eeda1e9dd51a56bf0783f427b4fcf5a1f489254
diff --git a/version.mk b/version.mk
index 8811741..ed67dee 100644
--- a/version.mk
+++ b/version.mk
@@ -108,7 +108,7 @@
 ifdef TARGET_BUILD_APPS
 ifeq ($(strip $(HAS_BUILD_NUMBER)),false)
   git_hash := $(shell git --git-dir $(LOCAL_PATH)/.git log -n 1 --pretty=format:%h)
-  date_string := $(shell date +%m%d%y_%H%M%S)
+  date_string := $$(date +%m%d%y_%H%M%S)
   version_name_package := $(base_version_major).$(base_version_minor).$(base_version_build) (eng.$(USER).$(git_hash).$(date_string)-$(base_version_arch)$(base_version_density))
 else
   version_name_package := $(base_version_major).$(base_version_minor).$(base_version_build) ($(BUILD_NUMBER_FROM_FILE)-$(base_version_arch)$(base_version_density))