Ramdisk: download, but don't add header, don't pass to u-boot
Currently, LAVA assumes that the ramdisk will always be passed to the kernel by u-boot. But, there are other ways for the DT to get to the kernel. Either using the initrd= commandline, or by adding properties into the DT that tell the kernel where the ramdisk is loaded in memory. The kernel also supports other fs formats besides .cpio.gz.
At least for u-boot devices, LAVA assumes that ramdisk will be .cpio.gz, but also always adds a u-boot header to the ramdisk image. (There is an 'add_header' paramter, but is it not override-able by device-types.) It also always passes the ramdisk addr as an argument to bootX.
Both of these steps (adding u-boot header, passing to bootx) should be options that can be overridden. Probably respecting "allow_modify" to avoid adding u-boot header is enough, but another flag to signal "don't pass this ramdisk via u-boot" is also needed.
Migrated from https://projects.linaro.org/browse/LAVA-1136