Add AB slot helpers

[learmj]

Install a udev rule to create aliases for the other (ie the inactive) slot partitions, and a script to assist with slot identification. Alias nodes are also present in the initramfs.

[learmj]

This allows us to do:

$ ls -l /dev/disk/by-label/
total 0
lrwxrwxrwx 1 root root 15 Jan 17 13:48 BOOTA -> ../../mmcblk0p2
lrwxrwxrwx 1 root root 15 Jan 17 13:48 BOOTB -> ../../mmcblk0p3
lrwxrwxrwx 1 root root 15 Jan 17 13:48 BOOTFS -> ../../mmcblk0p1
lrwxrwxrwx 1 root root 15 Jan 17 13:48 ROOTA -> ../../mmcblk0p4
lrwxrwxrwx 1 root root 15 Jan 17 13:48 ROOTB -> ../../mmcblk0p5
lrwxrwxrwx 1 root root 15 Jan 17 13:48 USERDATA -> ../../mmcblk0p6

$ slot                                                     
A                                                                            
$ slot other                                               
B                                                                            
$ ls -l /dev/disk/by-slot/other/
total 0
lrwxrwxrwx 1 root root 18 Jan 17 13:48 BOOT -> ../../../mmcblk0p3
lrwxrwxrwx 1 root root 18 Jan 17 13:48 ROOT -> ../../../mmcblk0p5
$
$ mount | grep -E 'mmcblk0p2|mmcblk0p3|mmcblk0p4|mmcblk0p5'
/dev/mmcblk0p4 on / type ext4 (rw,relatime,errors=remount-ro)                
/dev/mmcblk0p2 on /boot/firmware type vfat (rw,noatime,fmask=0022,dmask=0022,codepage=437,iocharset=ascii,shortname=mixed,errors=remount-ro)

sudo reboot '0 tryboot'

$ slot
B
$ slot other
A
$ ls -l /dev/disk/by-slot/other/
total 0
lrwxrwxrwx 1 root root 18 Jan 17 13:48 BOOT -> ../../../mmcblk0p2
lrwxrwxrwx 1 root root 18 Jan 17 13:48 ROOT -> ../../../mmcblk0p4
$
$ mount | grep -E 'mmcblk0p2|mmcblk0p3|mmcblk0p4|mmcblk0p5'
/dev/mmcblk0p5 on / type ext4 (rw,relatime,errors=remount-ro)
/dev/mmcblk0p3 on /boot/firmware type vfat (rw,noatime,fmask=0022,dmask=0022,codepage=437,iocharset=ascii,shortname=mixed,errors=remount-ro)