samples: guard sub-directories with CONFIG options

Do not descend to sub-directories when unneeded.

I used subdir-$(CONFIG_...) for hidraw, seccomp, and vfs because
they only contain host programs.

While we are here, let's add SPDX License tag, and sort the directories
alphabetically.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

samples/vfs/Makefile

@@ -1,5 +1,5 @@
 # List of programs to build
-hostprogs-$(CONFIG_SAMPLE_VFS) := \
+hostprogs-y := \
 	test-fsmount \
 	test-statx