Don't implement strtoul() on top of strtoull() as strtoull() is much
bigger and only used on linux currently. Instead refactor most of the
logic out of strtoul() into static inlines and reuse that. Also put it
in a separate object so it won't get linked in.

Signed-off-by: Piotr Jaroszyński <p.jaroszynski@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>