dhcp-client: fix invoke dhclient-script failed on Read-only file system

dhcp-client: fix invoke dhclient-script failed on Read-only file system

In read-only file system, '/etc' is on the readonly partition,
and '/etc/resolv.conf' is symlinked to a separate writable
partition.

In this situation, we should use shell variable to instead of
the temp file '/etc/resolv.conf.dhclient'.

[YOCTO #5624]
(From OE-Core rev: df793f4356b411cbb92445c4559c9b21eb6d99fc)

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>