Add sysconfdir to default systemd service substitutions

This could be specified as a custom substition by recipes that use it,
but like bindir, sbindir, and envfiledir, it seems common enough that it
should be a default substitution.

Signed-off-by: Xo Wang <xow@google.com>
Change-Id: I827aad06ca8401a22554b87499ba7f33470f438d