oniro-grub-bootconf: Respect KERNEL_IMAGETYPE as opposed to hardcoding bzImage

This patch makes sure that the resulting cfg takes into consideration
the kernel image type for the specific machine. This change makes now
the recipe/package machine specific so we drop allarch and set
PACKAGE_ARCH accordingly.

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>