build: Adding check_variables()

When building edeploy, it's mandatory to get absolute an absolute path
for TOP unless it will fail at deleting or umounting stuff.

This commit add a sanity check to avoid this mistake seen in the CI.
common: Fixing typo
Merge remote-tracking branch 'goneri/master'
tests: set ONFAILURE=console

This is better to debug.
cosmetic fixes in README.rst
general: avoid direct “rpm -i” invocation

yum maintains its own cache and the direct call of “rpm -i“ is enough
to get it outdated.
common: add TODO regarding cleanup() and pxe.install
Revert "makefile: use simplified targets"

This reverts commit 8e8fd55118a2c2f463c5f0fbe5d6f8f6e69fc464.

A phony target should not be a prerequisite of a real target file.
base: lsb-release is a debian package

redhat-lsb-core is the RHEL package that provides lsb_release