test/functional/aarch64: Split the ast2700a1-evb OpenBMC boot test

test/functional/aarch64: Split the ast2700a1-evb OpenBMC boot test

The 'ast2700a1-evb' machine has two functional tests: one loading
firmware components into memory and another using a vbootrom
image. Both tests perform a full OpenBMC boot and run checks on I2C
and PCIe devices, which is redundant and time-consuming.

To save CI resources, the vbootrom test is refactored to focus on the
firmware boot process only. The OpenBMC boot verification logic is
split and a new verify_openbmc_boot_start() helper is introduced to
only wait for the kernel to start.

The vbootrom test now uses this function and the less essential I2C
and PCIe checks have been removed from this test case.

Cc: Thomas Huth <thuth@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Jamin Lin <jamin_lin@aspeedtech.com>
Link: https://lore.kernel.org/qemu-devel/20251007141604.761686-6-clg@redhat.com
[ clg: Changed pattern from 'Starting kernel ...' to 'Linux version ' ]
Signed-off-by: Cédric Le Goater <clg@redhat.com>