[TESTS] Improve Guest NOR flash utilization

Currently, we have 16MB of NOR flash in all types of arm32 and arm64
Guests. The Guest NOR flash has: basic firmware (1M), linux kernel
image (6M), and rootfs (6M). This means we still have 3MB of Guest
NOR flash unused for almost all arm32 and arm64 Guests.

This patch reorganizes Guest NOR flash to have: basic firmware (1M),
linux kernel (8M), and rootfs (7M). This way we utilize complete
16MB of Guest NOR flash.

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