sim: simplify hardware m4 macro

Every port using this sets the 1st arg to yes and the 2nd arg to "".
These are the defaults we probably want anyways in order to unify the
codebase, so move them to the macro and only allow ports to declare
extra hardware models.

sim/mips/configure

@@ -12575,23 +12575,16 @@ sim_micromips_flags=" -F ${sim_micromips_filter}  ${sim_micromips_machine}  ${si
 #
 # Add simulated hardware devices
 #
-hw_enabled=no
+hw_extra_devices=""
 case "${target}" in
   mips*tx39*)
-	hw_enabled=yes
 	hw_extra_devices="tx3904cpu tx3904irc tx3904tmr tx3904sio"
 	;;
   *)
 	;;
 esac
 
-if test "$hw_devices"; then
-  hardware="$hw_devices"
-else
-  hardware="cfi core pal glue"
-fi
-hardware="$hardware $hw_extra_devices"
-
+hardware="cfi core pal glue $hw_extra_devices"
 sim_hw_cflags="-DWITH_HW=1"
 sim_hw="$hardware"
 sim_hw_objs="\$(SIM_COMMON_HW_OBJS) `echo $sim_hw | sed -e 's/\([^ ][^ ]*\)/dv-\1.o/g'`"
@@ -12600,7 +12593,7 @@ sim_hw_objs="\$(SIM_COMMON_HW_OBJS) `echo $sim_hw | sed -e 's/\([^ ][^ ]*\)/dv-\
 if test "${enable_sim_hardware+set}" = set; then :
   enableval=$enable_sim_hardware;
 else
-  enable_sim_hardware="$hw_enabled"
+  enable_sim_hardware="yes"
 fi
 
 case ${enable_sim_hardware} in