netlink: build a separate libsancus_netlink library

size comparison of 'libsancus.so' with and without netlink
------------------------------------------------------------------------------
 text    data     bss     dec     hex filename
26409     480       8   26897    6911 libsancus.so.0.0.0 <-- with netlink
18722     416       8   19146    4aca libsancus.so.0.0.0 <-- without netlink
------------------------------------------------------------------------------

size of the standalone 'libsancus_netlink.so'
------------------------------------------------------------------------------
 text    data     bss     dec     hex filename
 9173     360       8    9541    2545 libsancus_netlink.so.0.0.0
------------------------------------------------------------------------------

Note:

This commit also moves the netlink sources into a dedicated subdir, and the
netlink part will only be build if netlink support is detected by checking
for the 'linux/netlink.h' file.