Add Esp32 gdbstub Component and fix Esp8266 syscall errno handling

[mikee47]

Fix gdb_syscall return value, and set errno as appropriate

• The GDB reply packet is: ‘Fretcode,errno,Ctrl-C flag;call-specific attachment’
  The syscall function must return retcode NOT -errno.
  Take errno and set the global errno from it.

Add gdbstub Component for Esp32

• Enables LiveDebug sample to compile. We may also want to add serial debugging in the future (e.g. don't have JTAG adapter)

Add strerror_r implementation for old ESP8266 toolchain

• So we can get consistently error strings for errno (although as a legacy toolchain don't care about having a description)