…t to cmd.exe.

Some Windows development environments invoke a mingw version of gnu make
with SHELL set to /bin/sh or /bin/bash (either explicitly or by default),
in which case tinyUSB make fails when a $(shell...) command feeds DOS syntax to sh.
We can't just use sh, as many Windows environments won't have unix utilities
like realpath (used by this makefile for sh shell commands).

This fix forces SHELL=cmd.exe on Windows and documents the issue.
Also adds additional documentation to help the next person...

With this fix, tinyUSB can be easily built and debugged in environments like
NXP's MCUxpresso (import as a makefile project).