tracing: create trace2line

This lead to a re-factoring of count boot instructions, since trace2line
relies on the same tracing mechanism.

Also, now that we have common factored, I've decided to put the traces on
the out dir, to concentrate all outputs in a single place, and allow
easy switching between archs.

build

@@ -155,7 +155,10 @@ BR2_ROOTFS_POST_SCRIPT_ARGS=\"$post_script_args\"
 fi
 
 # Build.
-mkdir -p "${root_dir}/9p"
+mkdir -p \
+  "${root_dir}/9p" \
+  "${lkmc_out_dir}" \
+;
 cd "${buildroot_dir}"
 # HOST_QEMU_OPTS is a hack that happens to work because the QEMU package luckly uses += at all times.
 # It shouldn't be necessary in the first place: https://bugs.busybox.net/show_bug.cgi?id=9936
@@ -169,7 +172,7 @@ cmd="time \
   env \
   -u LD_LIBRARY_PATH \
   make \
-  O='$out_dir' \
+  O='${out_dir}' \
   HOST_QEMU_OPTS='--enable-debug --extra-cflags=-DDEBUG_PL061=1 --enable-trace-backends=simple $qemu_sdl' \
   V='$v' \
   $extra_make_args \