Infers custom vendor partitions.

Rather than hardcoding a list of allowed vendor partitions, we accept
anything in the vendor target files that is not a framework partition.

Also extend support for inferred misc_info keys when the device uses
SYSTEM/product or SYSTEM/system_ext.

Test: test --host releasetools_test
Test: Use to merge a device with a custom IMAGES/*.img in the vendor
      build, and SYSTEM/system_ext in the system build.
Bug: 225902565
Change-Id: I638c0f9c019357150516ea6c208ecd60c03c450f