build: add a OFFLINE var to Makefile
We have a build environment that restricts our internet connection and currently building fails due to the lint-md target needs to be able to install npm modules.
This commit suggests adding a OFFLINE variable to the Makefile which can be used like in the following examples:
$ env OFFLINE=true make -j8 test
$ make -j8 test OFFLINE=true
Checklist
-
make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
commit message follows commit guidelines
Affected core subsystem(s)
build