Mainline u-boot support

As mentioned in #4, it is possible to build and boot u-boot on the MS220-8P:

U-Boot 2019.10 (Apr 04 2020 - 09:26:23 +0000)

MSCC VCore-III MIPS 24Kec
Model: Luton26 PCB090 Reference Board
DRAM:  128 MiB
Loading Environment from SPI Flash... SF: Detected mx25l12805d with page size 256 Bytes, erase size 64 KiB, total 16 MiB

OK
In:    serial@10100000
Out:   serial@10100000
Err:   serial@10100000
Net:   Could not get PHY for miim-bus1: addr 0
Could not get PHY for miim-bus1: addr 1
Could not get PHY for miim-bus1: addr 2
Could not get PHY for miim-bus1: addr 3
Could not get PHY for miim-bus1: addr 4
Could not get PHY for miim-bus1: addr 5
Could not get PHY for miim-bus1: addr 6
Could not get PHY for miim-bus1: addr 7
Could not get PHY for miim-bus1: addr 8
Could not get PHY for miim-bus1: addr 9
Could not get PHY for miim-bus1: addr 10
Could not get PHY for miim-bus1: addr 11

Warning: switch@1010000 (eth0) using random MAC address - ee:6a:6f:da:b9:7a
eth0: switch@1010000
Hit any key to stop autoboot:  0 
luton # dhcp
BOOTP broadcast 1
BOOTP broadcast 2
BOOTP broadcast 3
BOOTP broadcast 4
BOOTP broadcast 5
BOOTP broadcast 6
BOOTP broadcast 7
BOOTP broadcast 8
BOOTP broadcast 9
BOOTP broadcast 10
BOOTP broadcast 11
BOOTP broadcast 12
BOOTP broadcast 13
DHCP client bound to address 192.168.0.51 (20779 ms)

There are however, some issues as the pcb090 reference implementation in u-boot does not match the hardware configuration of the MS220-8P:

system LEDs are wrong/do not match the behaviour of RedBoot from Meraki
MS220-8P lacks the second PHY used for Ethernet ports 9-16
uart output is lost when the kernel hands over to userspace

u-boot brings some really nice benefits to the switch:

change the kernel command line without recompiling
tftp boot the kernel without flashing
supported/expected by other firmwares (e.g. OpenWrt)

halmartin / meraki-builder

Mainline u-boot support #5