configure.ac: improve validation of some --enable-xxx options

configure.ac: improve validation of some --enable-xxx options

Catch an invalid option like "--enable-buspirate=rubbish".
Also mention all valid values in the help text for those options.

Change-Id: Ib0fb8904132d07cc5cde421aa816ca6971a08769
Signed-off-by: R. Diez <rdiezmail-openocd@yahoo.de>
Reviewed-on: https://review.openocd.org/c/openocd/+/8540
Reviewed-by: R. Diez <rdiez-2006@rd10.de>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Tested-by: jenkins