[TESTS] Improve copy and start_linux commands for arm32 basic firmware

The patch makes following changes to arm64 basic firmware:
1. Print time taken by copy command in usecs instead of nsecs for
simplicity and readability.
2. Rearrange parameters of start_linux_fdt command
3. Make 'initrd_addr' and 'initrd_size' parameters of start_linux
and start_linux_fdt commands optional for NFS-based rootfs or
Disk-based rootfs.
4. Update VExpress-A15 cmdlist to reflect changes in start_linux_fdt
command.

Signed-off-by: Anup Patel <anup@brainfault.org>