share/mk/: Use a variable to prefix recursive make(1) output

share/mk/: Use a variable to prefix recursive make(1) output

Piping the output to sed(1) didn't behave well with -Orecurse.  Using a
variable behaves well, keeping output synchronized.  The length of the
variable name is specific, so that `$(INFO_)` uses exactly 8 characters,
a tab.

Signed-off-by: Alejandro Colomar <alx@kernel.org>