Linux recipes are throwing some warnings

The warnings:

WARNING: /work/cip-core/recipes-kernel/linux/linux-cip_4.19.140-cip33.bb: Unable to get checksum for linux-cip SRC_URI entry ${MACHINE}_defconfig: file could not be found
WARNING: /work/cip-core/recipes-kernel/linux/linux-cip-rt_4.4.231-cip47-rt30.bb: Unable to get checksum for linux-cip-rt SRC_URI entry ${MACHINE}_defconfig: file could not be found
WARNING: /work/cip-core/recipes-kernel/linux/linux-cip_4.4.230-cip47.bb: Unable to get checksum for linux-cip SRC_URI entry ${MACHINE}_defconfig: file could not be found
WARNING: /work/cip-core/recipes-kernel/linux/linux-cip-rt_4.19.135-cip31-rt13.bb: Unable to get checksum for linux-cip-rt SRC_URI entry ${MACHINE}_defconfig: file could not be found

There is no guarantee that such a file exists, especially when using a different kernel recipe in the layer referencing this one.

@jan-kiszka