Fix builds with enforce_vintf_kernel + !kernel

If a build has PRODUCT_OTA_ENFORCE_VINTF_KERNEL_REQUIREMENTS
set (which is set for P+ builds) but no kernel is built,
the original code emits a warning, but it won't be able
to build target files package because it depends on
kernel_configs.txt and kernel_version.txt without any rules.

Properly clear BUILT_KERNEL_CONFIGS_FILE and BUILT_KERNEL_VERSION_FILE
so that it won't be depended on in this case.

Bug: 184128489
Test: TH + forrest

Change-Id: I4b8fbd117f114555cf606e3c3b48eb564dbe786b