Add support for OS-agnostic board (CI runner)

Adds support for detecting an OS-agnostic board per comment on https://github.com/adafruit/Adafruit_Blinka/pull/831. Once this PR is merged, the appropriate changes will be made to Adafruit_Blinka to support this variable via PlatformDetect instead of using FORCEBOARD/FORCECHIP

Test output:

Board Detection Test

Check that the Chip and Board IDs match your board and that this it is
correctly detecting whether or not it is a Linux board.

Chip id:  OS_AGNOSTIC
Board id:  OS_AGNOSTIC_BOARD

Linux Detection
---------------
Is this an embedded Linux system? False

Raspberry Pi Boards
-------------------
Is this a Pi 3B+? False
Is this a Pi 4B? False
Is this a 40-pin Raspberry Pi? False
Is this a Raspberry Pi Compute Module? False

Other Boards
-------------------
Is this a Siemens Simatic IOT2000 Gateway? False
Is this a 96boards board? False
Is this a BeagleBone board? False
Is this a Giant board? False
Is this a Coral Dev board? False
Is this a MaaXBoard? False
Is this a SiFive board?  False
Is this a PYNQ board? False
Is this a Rock Pi board? False
Is this a NanoPi board? False
Is this a Khadas VIM3 board? False
Is this a Clockwork Pi board? False
Is this a Seeed Board? False
Is this a UDOO board? False
Is this an ASUS Tinker board? False
Is this an STM32MP1 board? False
Is this a MilkV board? False
Is this a Luckfox Pico board? False
Is this a generic Linux PC? False
Is this an OS environment variable special case? False

@makermelissa - I noticed the "Is this an OS environment variable special case? False" test is False, but I do set this board within the os_environ_board() property. I've gone through debugging and need assistance here. Is there more to setting this property that I'm not seeing? Is this board not supposed to be the special case?

adafruit / Adafruit_Python_PlatformDetect

Add support for OS-agnostic board (CI runner) #353