oneplus-lemonade: Added device

Adds initial support for the Oneplus 9 (lemonade). Currently boots to init and supports USB networking. For the image to boot this requires a change to postmarketos-mkinitfs, I'll do this in a separate MR but wanted feedback on how I should do it. This should also work for the 9 pro (lemonadep), however might need a device specific dtbo like the empty-oneplus-lemonade.dts file with updated oem,project-id, oem,project-codename, and oem,hw-id.

The change to postmarketos-mkinitfs that is required, is to implement the deviceinfo_header_version variable that is listed in the wiki (but not implemented). mkbootimg also requires an optional parameter --dtb for the dtb to be loaded correctly by the bootloader - I assume adding a new variable deviceinfo_mkbootimg_dtb='/path/to/dtb' would be ok? Or should I use the existing deviceinfo_dtb variable and add something like deviceinfo_mkbootimg_include_dtb="true"?