Extracting kernel should not depend on system/vendor build.

If system and vendor are built separately, none of the two
builds contained kernel information. The process of extracting
kernel information shouldn't depend on system and vendor
builds, but on the existance of the kernel image.

With this change, one of system or vendor build may have
INSTALLED_KERNEL_IMAGE defined and the other has
PRODUCT_OTA_ENFORCE_VINTF_KERNEL_REQUIREMENTS defined. The one
with INSTALLED_KERNEL_IMAGE defined will contain kernel information
in target files.

After target files are merged, check_target_files_vintf will kick
in to do the checks properly.

Test: forrest
Bug: 180475190
Change-Id: I8d887dd94e1171ab277f02f25534caf098e3faae