Revert "base-files: source functions.sh in /lib/functions/system.sh"

Revert "base-files: source functions.sh in /lib/functions/system.sh"

This reverts commit 86aeac4fc98f42ac0ce7e0dcf1cb240e16b28f8f.

The reverted commit introduced a cyclic dependency between
/lib/functions.sh and /lib/functions/system.sh. Further details
are found in 282e8173509a ("base-files: do not source system.sh
in functions.sh"), which was applied to master some time ago and
is included in 21.02.

With the current age of 19.07 branch, it seems safer to revert this
mostly cosmetic feature than investing further time into disentangling
the dependencies.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>