Better support for creating user board targets

[sluubb]

When attempting to create a custom, project-specific board target for the XIAO RP2040 I found that it was very difficult to reuse parts from predefined boards that use the RP2040 chip. More specifically, it required a very roundabout way to setup the bootloader.

build.zig:

const std = @import("std");
const microzig = @import("microzig");

const MicroBuild = microzig.MicroBuild(.{
    .rp2xxx = true,
});

pub fn build(b: *std.Build) void {
    const mz_dep = b.dependency("microzig", .{});
    const mb = MicroBuild.init(b, mz_dep) orelse return;

    const pico_board = mb.ports.rp2xxx.boards.raspberrypi.pico.board orelse return;
    const target = mb.ports.rp2xxx.chips.rp2040.derive(.{
        .board = .{
            .name = "XIAO RP2040",
            .root_source_file = b.path("boards/xiao_rp2040.zig"),
            .imports = pico_board.imports,
        },
    });

    const firmware = mb.add_firmware(.{
        .name = "main",
        .target = target,
        .optimize = .ReleaseSmall,
        .root_source_file = b.path("src/main.zig"),
    });

    mb.install_firmware(firmware, .{});
}

boards/xiao_rp2040.zig:

const microzig = @import("microzig");
const hal = microzig.hal;

pub const xosc_freq = 12_000_000;

pub const bootrom = @import("shared/rp2040_bootrom.zig");

comptime {
    _ = bootrom;
}

pub const pin_config = hal.pins.GlobalConfiguration{
    .GPIO12 = .{ .name = "rgb_led", .direction = .out },
    .GPIO17 = .{ .name = "user_led_r", .direction = .out },
    .GPIO16 = .{ .name = "user_led_g", .direction = .out },
    .GPIO25 = .{ .name = "user_led_b", .direction = .out },
};

I find I'm having to reference the generated bootloader for rp2040 boards though other boards (here the rp pico) and copy over the rp2040_bootrom.zig to my project for it to compile, which feels convoluted. Is it possible to streamline this process by, for example, exposing the bootloader generation to the user?