I spent some time this weekend trying to reduce the boot times for my
pocketbeagle based things. A few things I discovered:1) The existence of an initrd in /boot adds 19 seconds to the boot. This
is seen in the dmesg logs where the "EXT4-fs (mmcblk0p1): mounted
filesystem" line is at the 20 second mark instead of the 1.1 second mark.
No idea why, but since the initrd isn't needed with the ext4 roots, just
deleting it is huge. Too bad update_kernel creates it.
This is only needed for kernels: 3.8 -> 4.3.x-ish (uuid mmc id) and
3.8 -> 4.1.x (kernel overlays)..
I'm thinking of nuking the initrd by default going forward..
2) generic-board-startup.service takes up a HUGE amount of time, over 40
seconds. I started debugging into it and 30 seconds of that is just calling
"systemctl start serial-getty@ttyGS0.service || true". Changing that to
"systemctl start serial-getty@ttyGS0.service &" drops boot time by almost 30
seconds. Is that something we could just "systemctl enable" and let systemd
start it if ttyGS0 is found and not have to deal with it in the
generic-board-startup? It's a lot of time saved.
Good point, no reason to not fork that. 
"config-pin -f pins.txt" also takes several seconds when configuring all 55
GPIO pins. I might look at optimizing that a bit for my use case by not
using config-pin and just use the /sys/class/gpio stuff directly in my own
script. Only a couple seconds though.
Regards,