arm64: renesas: Clean up default boot command

arm64: renesas: Clean up default boot command

The current default boot command does not respect the Linux kernel 2 MiB
alignment requirement, present on aarch64 [1]:

"
The Image must be placed text_offset bytes from a 2MB aligned base
address anywhere in usable system RAM and called there.
"

Adjust the boot command such, that it always places both Image and DT at
the nearest highest 2 MiB aligned offset. The DT is placed at lower 2 MiB
aligned address, the aarch64 Image is placed at the next higher 2 MiB
aligned address. Is is unlikely that a DT would be larger than 2 MiB on
these systems.

Replace use of hard-coded load addresses with generic ${loadaddr} aligned
using setexpr. This way, if user picks valid ${loadaddr}, their kernel and
DT address will be correctly set as well.

Fix up boot commands to use && instead of ; to exit the boot command early
in case of failure.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/arch/arm64/booting.rst#n138

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>