This patch introduce the lowpan_push_hc_data function to set data in
the iphc buffer.

It's a common case to set data and increase the buffer pointer. This
helper function can be used many times in header_compress function to
generate the iphc header.
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>