[settings] Allow numeric_setting_value() to handle long setting values

Allow numeric_setting_value() to handle e.g. the byte sequence

  00:00:00:00:12:34:56:78

by returning -ERANGE only if the value actually overflows the return
type.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
Merge branch 'master' into osimages