Correct parsing of rules.mk for keyboard revisions

Correct parsing of rules.mk for keyboard revisions

Previously, when looking for rules.mk files, we'd parse the individual
folders (A/B/C/D/E) into 5 variables, (A/B/C/D/E, A/B/C/D, A/B/C, A/B,
and A). Then, we'd get the final directory names and store _those_ in 5
new variables (A, B, C, D, and E). Then, when looking for the rules.mk,
we'd look in root_dir/keyboards/(A|B|C|D|E)/rules.mk, instead of looking
in root_dir/keyboards(A|A/B|A/B/C|A/B/C/D|A/B/C/D/E)/rules.mk. This
commit changes that logic from the former to the latter.