"zImage" not supported on arm64 so set KERNEL_IMAGETYPE_DIRECT to
"Image" for Raspberry Pi 5 and avoid the following errors:
| make: *** No rule to make target 'zImage'. Stop.
| ERROR: oe_runmake failed
This work was sponsored by GOVCERT.LU.
Signed-off-by: Leon Anavi <leon.anavi@konsulko.com>