[project] Clean up src/ tree

A long overdue cleanup of the src/ tree.

- Moved src/drivers to src/user because it contains more than drivers
- Removed src/drivers/ahci because it's unused - will restore it when I
  make a real AHCI driver
- Removed unused src/tools
- Moved kernel.ld (the only used file under src/arch) to src/kernel for
  now, if/when there's a multi-platform effort that should be figured
  out as part of it
- Removed the rest of the unused src/arch
- Renamed 'fb' to 'drv.uefi_fb' and 'nulldrv' to 'testapp'

src/boot/main.cpp

@@ -40,8 +40,8 @@ struct program_desc
 
 const program_desc program_list[] = {
 	{L"kernel", L"jsix.elf"},
-	{L"null driver", L"nulldrv.elf"},
-	{L"fb driver", L"fb.elf"},
+	{L"test application", L"testapp.elf"},
+	{L"UEFI framebuffer driver", L"drv.uefi_fb.elf"},
 };
 
 /// Change a pointer to point to the higher-half linear-offset version