rootfs.bdev_type: implement get and clear
rootfs.bdev_type: implement get and clear

clear is just set(x, ""), but before this patch that didn't work bc
"" is nota valid bdev type.

Signed-off-by: Serge Hallyn <3df611b026e4639bee8aef7a4beb2e39fcebb313@ubuntu.com>

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