Add support for dependent packages when using build.zig

[frmdstryr]

For #855

Example build.zig

const Builder = @import("std").build.Builder;
const Package = @import("std").build.Pkg;
const builtin = @import("builtin");


pub fn build(b: *Builder) void {
    // Cortex-M7 uses armv7em
    const arch = builtin.Arch{ .thumb = .v7em };
    const cpu = &builtin.Target.arm.cpu.cortex_m7;

    const mode = b.standardReleaseOptions();
    const elf = b.addExecutable("firmware.elf", "startup.zig");
    elf.setTheTarget(builtin.Target{
        .Cross = .{
            .arch = arch,
            .os = .freestanding,
            .abi = .none,
            .cpu_features = builtin.Target.CpuFeatures.initFromCpu(arch, cpu),
        },
    });

    const stm32 = Package{
        .name="stm32",
        .path="../../lib/stm32/index.zig"
    };
    elf.addPackage(stm32);
    elf.addPackage(Package{
        .name="gfx",
        .path="../../lib/gfx/tft.zig",
        .dependencies=&[_]Package{stm32},
    });

    elf.setBuildMode(mode);
    elf.setLinkerScriptPath("stm32h743xi.ld");
    elf.setOutputDir(b.build_root); // always build in same dir as build.zig
    elf.single_threaded = true;

    const gdb = b.addSystemCommand(&[_][]const u8{"arm-none-eabi-gdb"});
    gdb.addArtifactArg(elf);
    b.step("run", "Run with gdb").dependOn(&gdb.step);

    b.default_step.dependOn(&elf.step);
    b.installArtifact(elf);
}

[daurnimator]

        .path="../../lib/gfx/tft.zig",

I think we should point at the package's build.zig

And perhaps that means that build.zig needs to have some public function to prepare its own build options?

[andrewrk]

And perhaps that means that build.zig needs to have some public function to prepare its own build options?

See #353. That's part of a larger effort towards a package manager. This pull request is more exposing CLI options to the build script.

[andrewrk]

Thanks @frmdstryr