[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'

configs/custom.ninja

@@ -39,18 +39,18 @@ build ${build_root}/fatroot/jsix.elf : cp ${build_root}/jsix.elf
 build ${build_root}/fatroot/efi/boot/bootx64.efi : cp ${build_root}/boot/boot.efi
     name = bootloader to FAT image
 
-build ${build_root}/fatroot/nulldrv.elf : cp ${build_root}/user/nulldrv.elf
+build ${build_root}/fatroot/testapp.elf : cp ${build_root}/user/testapp.elf
     name = null driver to FAT image
 
-build ${build_root}/fatroot/fb.elf : cp ${build_root}/user/fb.elf
-    name = fb driver to FAT image
+build ${build_root}/fatroot/drv.uefi_fb.elf : cp ${build_root}/user/drv.uefi_fb.elf
+    name = UEFI framebuffer driver to FAT image
 
 build ${build_root}/fatroot/symbol_table.dat : makest ${build_root}/jsix.elf
 
 build ${build_root}/jsix.img : makefat | $
     ${build_root}/fatroot/symbol_table.dat $
-    ${build_root}/fatroot/nulldrv.elf $
-    ${build_root}/fatroot/fb.elf $
+    ${build_root}/fatroot/testapp.elf $
+    ${build_root}/fatroot/drv.uefi_fb.elf $
     ${build_root}/fatroot/jsix.elf $
     ${build_root}/fatroot/efi/boot/bootx64.efi
     name = jsix.img

configs/debug.toml

@@ -79,7 +79,7 @@ cxxflags = [
 
 ldflags = [
 	"${ldflags}",
-	"-T", "${source_root}/src/arch/x86_64/kernel.ld",
+	"-T", "${source_root}/src/kernel/kernel.ld",
     "-g",
     "-nostdlib",
     "-Bstatic",