Document SPL build options

[maxgerhardt]

The builder script has the option to enable or disable the discovery of built-in SPL libraries

platform-gd32/builder/frameworks/spl.py

Lines 163 to 164 in e7811ac

	should_include_libs = board.get("build.spl_libs", True)
	should_include_libs = str(should_include_libs).lower() in ("1", "yes", "true")

A further work-in-progress is that linking against semihosting-enabled libraries can be done

platform-gd32/builder/frameworks/spl.py

Lines 39 to 40 in e7811ac

	activate_semihosting = board.get("debug.semihosting", False)
	activate_semihosting = str(activate_semihosting).lower() in ("1", "yes", "true")

All of this needs to be documented and explained properly for end users.