Merge branch 'L2SS-290-Add_lofar20rc_check_to_Makefile' into 'master'

L2SS-290: Add a check to the Makefile that the lofar20rc.sh file has been sourced

Closes L2SS-290

See merge request !69

docker-compose/Makefile

+# Before doing anything, make 100% certain that all necessary environment
+# variables are set.
+ifndef LOFAR20_DIR
+    # Generate the full path to the lofar20rc.sh file
+    # for exactly this repository.
+    LOFAR20RC = $(subst docker-compose,bootstrap/etc/lofar20rc.sh,$(abspath .))
+    $(error There are essential LOFAR2.0 environment variables missing. You must source the lofar20rc.sh file of this repo first. You can do it like this (paste without quotes): ". $(LOFAR20RC)")
+endif
 # Set dir of Makefile to a variable to use later
 MAKEPATH := $(abspath $(lastword $(MAKEFILE_LIST)))
 BASEDIR := $(notdir $(patsubst %/,%,$(dir $(MAKEPATH))))