Makefile: Correct and expand translation excluded directories
Makefile: Correct and expand translation excluded directories

As originally written, the TRANSLATE_SOURCES_EXC was incorrect.  If
more than one build directory existed, "make translate"
(and make check-translate) would error.

I corrected the problem, commented it, and added a number of additional
exclude directives.

I reviewed the PR and should have caught this, but did not.

adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython