[fix] brands: add variables from build env to grunt process

We have some variables in the build environment which are also needed in the
grunt process when building themes.  Theses variables are relavant if one
creates a fork with its own branding.  We treat these variables under the term
'brands'.

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

Makefile

@@ -28,6 +28,7 @@ help:
 	@echo  '  gh-pages  - build docs & deploy on gh-pages branch'
 	@echo  '  clean     - drop builds and environments'
 	@echo  '  project   - re-build generic files of the searx project'
+	@echo  '  themes    - re-build build the source of the themes'
 	@echo  ''
 	@$(MAKE) -s -f utils/makefile.include make-help
 	@echo  ''
@@ -87,6 +88,20 @@ searx.brand:
 	$(Q)echo "DOCS_URL = '$(DOCS_URL)'" >> searx/brand.py
 	$(Q)echo "PUBLIC_INSTANCES = 'https://searx.space'" >> searx/brand.py
 
+# build themes
+# ------------
+
+PHONY += themes themes.oscar themes.simple
+themes: themes.oscar themes.simple
+
+themes.oscar:
+	$(Q)echo '[!] Grunt build : oscar theme'
+	$(Q)grunt --gruntfile  "searx/static/themes/oscar/gruntfile.js"
+
+themes.simple:
+	$(Q)echo '[!] Grunt build : simple theme'
+	$(Q)grunt --gruntfile  "searx/static/themes/simple/gruntfile.js"
+
 # test
 # ----