Specify cmdline property in autogen (vendor_)boot.img

Value of the `--(vendor_)cmdline` is `INTERNAL_KERNEL_CMDLINE` in make.
This is passed as an argument to mkbootimg invocation of boot.img when
vendor_boot.img is not created, and always passed as an argument to
vendor_boot.img.

Test: inspect autogen vendor_boot.img ninja command
Bug: 379945468
Change-Id: Id25d33b970e028c2fe66c73d66c2091a91f56666