shave off 5544 bytes with LTO

[H3wastooshort]

I have messed around with the original MeshCore firmware, trying to fit it into the PSoC4's memory. I that process, I have managed to make Link-Time Optimization work, by marking some internal functions as "used", so they don't get removed. This saves about 5kB on this project, but needs my slightly modified fork of CubeCell-Arduino. Feel free to fork my fork, if you intend to merge this.

before:

RAM:   [=====     ]  52.4% (used 8584 bytes from 16384 bytes)
Flash: [==========]  99.5% (used 130432 bytes from 131072 bytes)

with LTO:

RAM:   [=====     ]  51.6% (used 8448 bytes from 16384 bytes)
Flash: [==========]  95.3% (used 124888 bytes from 131072 bytes)

As i have not tested these changes in production, I am keeping this a Draft for now.

[H3wastooshort]

testing on real hardware sadly results in a bootloop with LTO enabled.
my CubeCell-Arduino Fork also strips out some unused SX1262 driver code though. This shaves off 520 bytes of flash.

stock CubeCell-Arduino

RAM:   [=====     ]  52.4% (used 8584 bytes from 16384 bytes)
Flash: [==========]  99.6% (used 130504 bytes from 131072 bytes)

my fork

RAM:   [=====     ]  51.3% (used 8408 bytes from 16384 bytes)
Flash: [==========]  99.2% (used 129984 bytes from 131072 bytes)