2.1.4 - Major Serial improvements, lots of significant fixes!

Right on the heels of 2.1.0, we have some changes that initially started as the small addition of U2X support to Serial, because I happened to need it... but before I knew it, I'd reworked that class in a major way... Flash use is significantly lower, particularly on small chips, where the OSCERR sigrow correction is dropped to save a significant amount of flash (it was never truly necessary anyway - but we still do it on parts with more than 4k flash, where we aren't hurting for every instruction word of flash we can get our hands on) - we also no longer apply the correction when using external clock, as in that case it is an incorrection, not a correction... It also no longer messes with interrupt priority, saving a significant amount of flash and preventing a few hairy situations. In my tests, it seems to work. A critical issue with 2.1.x that broke alternate serial pins is also fixed now. Version defines should be wrong less often moving forward, as that logic has been moved to one place in platform.txt. the build.board changes that caused some problems in the previous release now have backwards compatibility defines (though we strongly recommend testing for MEGATINYCORE, not the ARDUINO_attinyxy* defines). All options are now available for SUT, now too. A few other boards.txt bugs are also fixed. I had a chance to try out the external clock - I could hit 32 MHz with it, even at surprisingly low voltages, though glitches on the voltage could break it. at the lower end of supply voltages! Oh - and users of I2C slave mode will be happy to hear that we have a fix for a timing issue thanks to @raspy135 - they encountered it when using an ESP32 as the master.

So get this loaded up and start reporting all the new bugs I have surely introduced!
This is the version for board manager installation.