ifupdown-script: send current hostname in DHCP request

ifupdown-script: send current hostname in DHCP request

For the default (BR2_SYSTEM_DHCP) /etc/network/interfaces handling.

Both the "big" and busybox ifupdown variants pass the hostname argument to
the DHCP client through the shell, so we can use $() to expand to the
current hostname value.

Notice that this may not be handled by all DHCP clients.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Reviewed-by: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>