Generate images for 512B and 4096B sector sizes

[lool]

Splits image generation in two steps, and generate images for 512B and 4096B sector sizes.

[lool]

We discussed this with Nicolas, he suggested using 4Kn to name the image with a 4k sector size, inspired by Advanced Format, and I've proposed calling the two images -sdcard and -ufs which he is ok with.

[lool]

I've manually downloaded and checked the resulting disk images as the build output didn't make it clear whether 4k or 512B sector size was being used. The resulting files were correct

$ fdisk -b 512 -l disk-sdcard.img
Disk disk-sdcard.img: 4 GiB, 4294967296 bytes, 8388608 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: 784CEFE1-B4B7-484B-9EE6-8BF49866E064

Device             Start     End Sectors  Size Type
disk-sdcard.img1    8192 1056767 1048576  512M EFI System
disk-sdcard.img2 1064960 8388574 7323615  3.5G Linux root (ARM-64)


$ fdisk -b 4096 -l disk-ufs.img
Disk disk-ufs.img: 4 GiB, 4294967296 bytes, 1048576 sectors
Units: sectors of 1 * 4096 = 4096 bytes
Sector size (logical/physical): 4096 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: gpt
Disk identifier: 8DFCB174-46FF-471F-BB4F-3A971BBCDF78

Device         Start     End Sectors  Size Type
disk-ufs.img1   1024  132095  131072  512M EFI System
disk-ufs.img2 133120 1048570  915451  3.5G Linux root (ARM-64)