Replace product variable stashing with .KATI_READONLY

Instead of using rot13 / rot26, use the Kati extension to mark these
variables as readonly.

Move $(strip) for a few variables to before they're marked readonly. Use
a different variable for modifications to BOARD_KERNEL_CMDLINE in
build/core/Makefile.

Test: build/tools/kati_all_products.mk on AOSP and internal master
Test: build-aosp_bullhead.ninja the same before / after
Change-Id: If98b24af763831a9c5c2de38037a69ab1bf9e023