package: Drop subshell usage for dwarfsrcfile generation.

package: Drop subshell usage for dwarfsrcfile generation.

The command for running dwarfsrcfiles is simple and does not need a subshell
for each execution. By expanding out this function to use check_output()
from subprocess and a list of arguments, the shell overhead can be dropped.

For recipes with lots of files this gives a significant saving.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>