Do not assume GKI just with vendor_boot.

This change ensures changes to GENERIC_KERNEL_CMDLINE only affects
devices that explicitly says it uses GKI/generic boot image.

In details, if the device has vendor_boot, but does not explicitly
specify that it uses GKI/generic boot image, do not include
GENERIC_KERNEL_CMDLINE in boot. boot cmdline is left empty
in this case.

The old logic:
- If device uses GKI *OR* has vendor_boot:
  boot uses GENERIC_KERNEL_CMDLINE, and do not include kernel base
  and pagesize.
  - If device has vendor_boot, INTERNAL_KERNEL_CMDLINE, kernel base
    and pagesize goes in vendor_boot.
- If device does not use GKI nor have vendor_boot:
  boot uses INTERNAL_KERNEL_CMDLINE, and includes kernel base and
  pagesize.

The new logic:
- If using GKI, boot uses GENERIC_KERNEL_CMDLINE. Remove kernel base
   and pagesize because they are device-specific.
- If not using GKI:
  - If building vendor_boot, INTERNAL_KERNEL_CMDLINE, base and
    pagesize goes in vendor_boot; boot does not have cmdline, base or
    pagesize.
  - Otherwise, put them in boot

Comparison of the code before and after:
- If device uses GKI,
  - For boot partition:
    - cmdline continues to be GENERIC_KERNEL_CMDLINE
    - kernel base and pagesize continues to be excluded
  - For vendor_boot partition:
    - cmdline continues to be INTERNAL_KERNEL_CMDLINE
    - kernel base and pagesize continues to be included
- If device does not use GKI:
  - If device has a vendor_boot partition:
    - For boot partition:
      * cmdline changes from GENERIC_KERNEL_CMDLINE to empty
      - kernel base and pagesize continues to be excluded
    - For vendor_boot partition:
      - cmdline continues to be INTERNAL_KERNEL_CMDLINE
      - kernel base and pagesize continues to be included
  - If device does not have a vendor_boot partition:
    - For boot partition:
      - cmdline continues to be INTERNAL_KERNEL_CMDLINE
      - kernel base and pagesize continues to be included

Test: builds
Bug: 171512004

Change-Id: I4feac435698f43ac299b430bff66147057865a62