sim: fix handling of 2nd arg to SIM_AC_OPTION_HARDWARE

sim: fix handling of 2nd arg to SIM_AC_OPTION_HARDWARE

The 2nd arg to SIM_AC_OPTION_HARDWARE is described as "a space separated
list of devices that override the defaults" while the 3rd arg is "a space
separated list of extra target specific devices".  But the macro doesn't
seem to treat the 2nd arg this way.

Instead, it will always add the default list of devices, and only add the
extra target specific devices if the 2nd arg is not specified.  So rework
the logic slightly to handle the 2nd arg as documented.

This shouldn't affect any targets in the tree as no one passes in a non-
empty value as the 2nd arg.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>