The timeout definition is usually handled dynamically in wic. For the
wic files using oniro-grub-bootconf as grub configuration, we need to
manually handle that via the existing WIC_BOOTLOADER_TIMEOUT variable.
In this way, WIC_BOOTLOADER_TIMEOUT can be used with oniro-grub-bootconf
and wic dynamic grub configuration (bootloader argument).

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