Add system_rpi0w target (Raspberry Pi Zero W)

[bchassoul]

Summary

Adds system_rpi0w as a new grisp_alloy target for the Raspberry Pi Zero W (BCM2835, ARMv6). Scaffolding, toolchain, SDK, fwup pipeline, USB-OTG console, Wi-Fi/Bluetooth/ramoops bring-up, and design docs.

Test plan

• ./build-toolchain.sh rpi0w produces the ARMv6 toolchain tarball.
• ./build-sdk.sh rpi0w builds; /lib/firmware/brcm/ in the rootfs contains brcmfmac43430-sdio.*.
• ./build-firmware.sh -i rpi0w hello_grisp produces .fw + .img.
• Flash image, attach USB-OTG, reach BEAM prompt over serial.
• On device: wlan0 present (ip link), /sys/class/bluetooth/hci0 exists, /sys/fs/pstore mounted.
• On device: all expected driver modules are live (cat /proc/modules lists at minimum brcmfmac, brcmutil, cfg80211, hci_uart, btbcm, bluetooth) and wc -l /lib/modules/*/modules.dep reports > 0.

note: if you use virtualbox, run the pipeline with VAGRANT_DEFAULT_PROVIDER=virtualbox

[ziopio]

@bchassoul super cool! And thank you for having inscluded so much documentation in the PR! I will test it with my Pi Zero 👍