amazon-soho: new device (and main/postmarketos-mkinitfs: change to enable postprocess scripts)

As seen in the comments in my previous attempt at a MR (!1995 (closed)), this new device requires a hack to the boot.img, and discussion led to the proposing of 'postprocess' scripts that could amend the boot.img as necessary after its creation. This MR includes the commits for both the device and kernel package for amazon-soho and the amendment of postmarketos-mkinitfs to enable such postprocess scripts to function. Thanks.