stlink: simplify the use of deprecated HLA transport

stlink: simplify the use of deprecated HLA transport

Commit 34ec5536c0ba ("stlink: deprecate HLA support") makes hard
to use the still functional HLA transport with the stlink listed
in board config files.
Now that the prefixes 'hla_' and 'dapdirect_' has been dropped
from the transport name, allow overriding the transport by using
the 'stlink-hla' script in front of the board file, e.g.:

	openocd -f interface/stlink-hla.cfg -f board/st_nucleo_f4.cfg

Revert the documentation changes of the change above.
Improve the documentation to explain how to use the compatibility
HLA mode.
Improve the error message in stlink driver to guide the user to
update the stlink firmware and to use the compatibility HLA mode.

Change-Id: I5d0bc7954511692cebe024bda2aaa72767b97681
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8679
Tested-by: jenkins