Add Gravitech Cucumber M, MS, R, and RS boards

[anecdata]

I couldn't make edits on PR #4131, so this is a replacement. Many thanks to @cotdp for all of the prior work.

Functionally, these boards are identical to the Saola boards ("M" is WROOM and "R" is WROVER), except they have an added LED, and the "S" versions have I2C sensors onboard that lock down the pins for the default I2C. Form factor differs from Saola, and the Cucumbers have a second USB-C connector for the USB.

For this PR, I started with the Saola WROOM (for M and MS) and WROVER (for R and RS) files, customized for the Gravitech boards, bringing in some of the code from @cotdp's commits on #4131.

Tested the R build on a Cucumber R, and the RS build on a Cucumber RS.

Limited testing of the M and MS builds on a Saola WROOM board. I think it's low-risk to merge, easy enough to file an issue or PR later.

code.py test results for the RS board:

MCU Gravitech Cucumber RS with ESP32S2
VER 7.0.0-alpha.5-98-g3ec3e7081-dirty on 2021-08-04
MEM 2043408
IDF TOT   155748
IDF FREE  131004
IDF LARG  65536
Sleep Memory: 4096
CIRPY83RIS / (512, 512, 1881, 869, 869, 0, 0, 0, 0, 255)
NVM: 20480
I2C addresses: 0x5f 0x68 0x76 

I'm open to adding more pin aliases, but it's not clear to me where to draw the line. The physical board uses only pin numbers, other than TX and RX, on the silk and on the labeled pin headers. The pin headers are color-coded, and Gravitech docs, posted by @skieast #4131 (comment) map the color-codings to various pin aliases for touch, SPI, ADC, DAC, USB, and an additional UART. Some of this is somewhat arbitrary. Touch pins are fixed, ADC are fixed, DAC are fixed though one is used by the NeoPixel. I2C pins are arbitrary on the non-"S" boards. The is_hardware_spi sample code yields 44,100 possible pin combinations for hardware SPI.

The current commit adds only the named aliases LED, NEOPIXEL, TX, RX, and UART on all 4 boards, and SCL, SDA, and I2C, on the 2 "S" (sensor) boards. More pin aliases can be added easily if desired.

[tannewt]

Looks good! Thank you!