gem5: track build and run variants separately with -M and -N

Otherwise, checking out branches is too insane, as it does not
update the worktrees, even though the gem5/gem5 module was updated.

gem5: expose build types, document debug builds.

simultaneous runs: store stdout and stderr on a file to allow running
all from a single terminal on the background cleanly.

build

@@ -21,7 +21,7 @@ post_script_args=
 qemu_sdl='--enable-sdl --with-sdlabi=2.0'
 suffix=
 v=0
-while getopts 'a:B:b:C:c:fGgj:hIiK:kL:lM:p:qSs:v' OPT; do
+while getopts 'a:B:b:C:c:fGgj:hIiK:kL:lM:p:qSst::v' OPT; do
   case "$OPT" in
     a)
       arch="$OPTARG"
@@ -98,6 +98,9 @@ BR2_TARGET_ROOTFS_INITRAMFS=n
     s)
       suffix="$OPTARG"
       ;;
+    t)
+      common_gem5_build_type="$OPTARG"
+      ;;
     v)
       v=1
       ;;
@@ -220,8 +223,9 @@ env \\
 make \\
 O='${buildroot_out_dir}' \\
 HOST_QEMU_OPTS='--enable-debug --enable-trace-backends=simple ${qemu_sdl}' \\
-GEM5_LKMC_SRCDIR="$common_gem5_src_dir" \\
+GEM5_LKMC_GEM5_BUILD_TYPE="$common_gem5_build_type" \\
 GEM5_LKMC_OUTDIR="$common_gem5_out_dir" \\
+GEM5_LKMC_SRCDIR="$common_gem5_src_dir" \\
 V='${v}' \\
 ${extra_make_args} \
 all \\