Added options to compile stdlib and init executable.

targets/x86_64/SConscript

@@ -42,7 +42,6 @@ prog_kernel = kernel_env.Program(
     source = kernel_sources
 )
 kernel_env.Depends(prog_kernel, [crti_o, crtn_o] + kernel_sources)
-kernel_env.Default(prog_kernel)
 
 x86_64_iso_files = [
     {
@@ -80,7 +79,6 @@ prog_loader = uefi_env.Program(
     target = loader_target,
     source = loader_sources
 )
-uefi_env.Default(prog_loader)
 
 ### Bootable Image
 def runcmd(*args, **kwargs):
@@ -125,7 +123,8 @@ cmd_iso = env.Command(
     source = [cmd_image],
     action = build_iso
 )
-env.Default(cmd_iso)
+alias_os = env.Alias('os', [cmd_iso])
+env.Default(alias_os)
 
 # finally update the environment
 env.Append(ISO_FILES = x86_64_iso_files)