Fix build and boot with modern GCC

[tuckerwales]

Summary

openiBoot failed to build and boot when compiled with modern arm-none-eabi-gcc (GCC 15) on Apple Silicon. This PR:

• Replaces SCons with GNU Make - SCons required Python 2 and had bit-rotted. The new GNUmakefile builds all targets with a single make command.
• Fixes the root cause of boot failures - GCC 15's strict handling of attribute((packed)) generated byte-by-byte LDRB sequences for naturally word-aligned structs, breaking interrupt timing and the cooperative task scheduler. Removed unnecessary packed from
  LinkedList, TaskRegisterState, and TaskDescriptor (layout verified identical via _Static_assert).
• Fixes ARM/Thumb interwork with GCC 15 - Added .type %function annotations to all ARM-mode assembly functions so the linker generates correct interwork veneers.
• Fixes driver failures - Rewrote SPI to polled I/O (interrupt handler missed timing windows); removed DMA interrupt registration that raced with polled completion; removed duplicate audiohw_init() call; fixed C11 inline semantics and task init ordering.
• Adds a default boot menu when /boot/menu.lst is missing, instead of silently failing.
• Adds CI - GitHub Actions workflow builds all targets and creates releases on tags.
• Adds documentation - Comprehensive README rewrite and new INSTALLING.md.

Changes by area

Build system

• Replaced SCons (SConstruct + 18 SConscript files) with a single GNUmakefile
• Supports all release, debug, and installer targets
• Removed -fPIC (not meaningful for bare-metal), changed -O2 to -O1
• Added -no-pie to mk8900image host tool link (fixes build with modern Ubuntu PIE defaults)
• Added .github/workflows/build.yml - builds all targets on push to tags, uploads artifacts, creates GitHub Release

Core fixes

• includes/openiboot.h: Removed attribute((packed)) from LinkedList, TaskRegisterState, TaskDescriptor - root cause of boot failure with GCC 15
• arch-arm/asmhelpers.sx: Added .type %function to all .code 32 functions for correct ARM/Thumb interwork veneers
• framebuffer.c: Changed inline to static inline (C11 semantics fix)
• tasks.c: Moved CurrentRunning assignment before task_init() to prevent NULL dereference in EnterCriticalSection

Driver fixes

• plat-s5l8900/spi.c: Rewrote from interrupt-driven to polled I/O
• plat-s5l8900/dma.c: Removed DMA interrupt registration that raced with polled dma_finish()
• plat-s5l8900/s5l8900.c: Removed duplicate audiohw_init() call from platform_init()

Menu

• menu/menu.c: When /boot/menu.lst is not found, creates a default menu with an "iOS" auto-boot entry instead of dropping to a blank screen. Console is always available as the last menu entry.

Documentation

• README.md: Comprehensive rewrite - supported devices, build instructions, menu configuration, project structure, architecture notes
• INSTALLING.md (new): Installation guide for Bootlace, manual recovery mode, DFU mode, and restore

Test plan

• Built all S5L8900 targets (iPhone2G, iPhone3G, iPodTouch1G) with arm-none-eabi-gcc 15.1 on Apple Silicon Mac
• Loaded iPhone3G image via recovery mode - boots to menu, USB console connects
• Verified default menu appears when /boot/menu.lst is absent
• Verified "Console" entry opens interactive console with working commands
• CI workflow builds all targets on Ubuntu runner

[nickpack]

Wow, I can't say I was expecting to ever see this build on a modern system!

It'll take me some time to consume this properly, and I am already pondering how I test it - I don't have any of the appropriate devices anymore - maybe one of the other guys who originally worked on it will have a device stashed somewhere that can build and test properly.

[tuckerwales]

I've only managed to test it on an iPhone 3G as I don't have the other devices either, but all works fine there!