kubecross: remove Makefile.build-image dependencie and check if the env vars are set

[cpanato]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

Follow up for #1943

remove the Makefile.build-image script dependency and perform environment check.
For now, it checks if the REGISTRY is set

/assign @justaugustus @saschagrunert @hasheddan

Which issue(s) this PR fixes:

None

Special notes for your reviewer:

Does this PR introduce a user-facing change?

NONE

[cpanato]

examples:

$ make container
Makefile:48: *** REGISTRY is undefined, please set to registry you want to push.  Stop.

and

$ REGISTRY=test/honk make container
./../../../hack/init-buildx.sh
echo "Building kube-cross for the following platforms: linux/amd64 linux/arm64 linux/ppc64le "
Building kube-cross for the following platforms: linux/amd64 linux/arm64 linux/ppc64le
Starting build for linux/amd64 platform
#1 [internal] load build definition from Dockerfile
#1 sha256:9f2685b62d868cedf4dd8f1c397bd7344b11d01ec8c9766053d135a0ff3d9c61
#1 transferring dockerfile: 32B done
#1 DONE 0.0s

#2 [internal] load .dockerignore
#2 sha256:5cda1b93467aa46a9900e409663e48d24445c333a6e32668cd00ae747d875646
#2 transferring context: 2B done
#2 DONE 0.0s

[hasheddan]

/lgtm

[k8s-ci-robot]

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