wic/qemuarm64: enabled kernel arguments with u-boot

Kernel commandline arguments passed via runqemu don't
configure the kernel when booting with u-boot, as it finds
configuration in extlinux/extlinux.conf instead, which is
generated by wic.

A custom extlinux.conf file is now passed to wic with the
intended arguments, so that the kernel now configures
as directed.

Change-Id: Icbbcab47fe5cf70e4d8d39c7145371f1d6cd8db7
Signed-off-by: Brett Warren <brett.warren@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>