package/openpowerlink: avoid kernel header issue with musl

The Virtual Ethernet driver doesn't build when the musl libc is used on the
system. As stated in the musl wiki [1], the userspace and kernel headers are
mixed leading to a "clash" with the definitions provided by musl.

Remove netinet/if_ether.h userspace header and replace ETHER_ADDR_LEN by
ETH_ALEN [2] and ETHERMTU by ETH_DATA_LEN [3] in veth-linuxuser.c.

Fixes:
http://autobuild.buildroot.org/results/2ca/2ca04bb046263e479e7597867b56469893d3c11d/

Upsteam status: pending
https://github.com/OpenAutomationTechnologies/openPOWERLINK_V2/pull/120

[Rebase on v2.2.2]
[1] http://wiki.musl-libc.org/wiki/FAQ#Q:_why_am_i_getting_.22error:_redefinition_of_struct_ethhdr.2Ftcphdr.2Fetc.22_.3F
[2] https://git.musl-libc.org/cgit/musl/tree/include/net/ethernet.h?h=v1.1.14#n35
[3] https://git.musl-libc.org/cgit/musl/tree/include/net/ethernet.h?h=v1.1.14#n48



Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>