Mini-OS: netfront: fix off-by-one error introduced in 7c8f3483

Mini-OS: netfront: fix off-by-one error introduced in 7c8f3483

7c8f3483 introduced a break within a loop in netfront.c such that
cons and nr_consumed were no longer always being incremented. The
offset at cons will be processed multiple times with the break in
place.

This commit reverts to using the "some" variable in the loop condition,
but avoids ifdefs for the non-libc case. It also renames it to dobreak
to make its usage clearer.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Tested-by: Sarah Newman <srn@prgmr.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>