semihosting/config: Merge --semihosting-config option groups

semihosting/config: Merge --semihosting-config option groups

Currently we mishandle the --semihosting-config option if the
user specifies it on the command line more than once. For
example with:
 --semihosting-config target=gdb --semihosting-config arg=foo,arg=bar

the function qemu_semihosting_config_options() is called twice, once
for each argument.  But that function expects to be called only once,
and it always unconditionally sets the semihosting.enabled,
semihost_chardev and semihosting.target variables.  This means that
if any of those options were set anywhere except the last
--semihosting-config option on the command line, those settings are
ignored.  In the example above, 'target=gdb' in the first option is
overridden by an implied default 'target=auto' in the second.

The QemuOptsList machinery has a flag for handling this kind of
"option group is setting global state": by setting
 .merge_lists = true;
we make the machinery merge all the --semihosting-config arguments
the user passes into a single set of options and call our
qemu_semihosting_config_options() just once.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Luc Michel <luc@lmichel.fr>
Message-id: 20220526190053.521505-3-peter.maydell@linaro.org