dhcpcd: do not pass --target option

dhcpcd is using $(BR2_GCC_TARGET_ARCH) as the value of its --target
option. However, depending on the architecture, this value might be
empty.

After inspecting the dhcpcd configure script, it turns out that in
fact the --target option has the following semantic:

  --target=TARGET   configure for building compilers for TARGET [HOST]

So it is very much like the --target option of regular,
autoconf-based, configure scripts. Since dhcpcd is not a compiler,
passing --target is pointless. Moreover, as long as --os=linux is
passed, passing --host or --target is completely ignored by the
configure script.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>