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/Makefile

+# SPDX-License-Identifier: GPL-2.0
 # Makefile for Linux samples code
 
-obj-y			+= kobject/ kprobes/ trace_events/ livepatch/ \
-			   hw_breakpoint/ kfifo/ kdb/ hidraw/ rpmsg/ seccomp/ \
-			   configfs/ connector/ v4l/ trace_printk/ \
-			   vfio-mdev/ vfs/ qmi/ binderfs/ pidfd/
+obj-$(CONFIG_SAMPLE_ANDROID_BINDERFS)	+= binderfs/
+obj-$(CONFIG_SAMPLE_CONFIGFS)		+= configfs/
+obj-$(CONFIG_SAMPLE_CONNECTOR)		+= connector/
+subdir-y				+= hidraw
+obj-$(CONFIG_SAMPLE_HW_BREAKPOINT)	+= hw_breakpoint/
+obj-$(CONFIG_SAMPLE_KDB)		+= kdb/
+obj-$(CONFIG_SAMPLE_KFIFO)		+= kfifo/
+obj-$(CONFIG_SAMPLE_KOBJECT)		+= kobject/
+obj-$(CONFIG_SAMPLE_KPROBES)		+= kprobes/
+obj-$(CONFIG_SAMPLE_LIVEPATCH)		+= livepatch/
+subdir-y				+= pidfd
+obj-$(CONFIG_SAMPLE_QMI_CLIENT)		+= qmi/
+obj-$(CONFIG_SAMPLE_RPMSG_CLIENT)	+= rpmsg/
+subdir-$(CONFIG_SAMPLE_SECCOMP)		+= seccomp
+obj-$(CONFIG_SAMPLE_TRACE_EVENTS)	+= trace_events/
+obj-$(CONFIG_SAMPLE_TRACE_PRINTK)	+= trace_printk/
+obj-$(CONFIG_VIDEO_PCI_SKELETON)	+= v4l/
+obj-y					+= vfio-mdev/
+subdir-$(CONFIG_SAMPLE_VFS)		+= vfs

samples/seccomp/Makefile

 # SPDX-License-Identifier: GPL-2.0
 ifndef CROSS_COMPILE
-hostprogs-$(CONFIG_SAMPLE_SECCOMP) := bpf-fancy dropper bpf-direct user-trap
+hostprogs-y := bpf-fancy dropper bpf-direct user-trap
 
 HOSTCFLAGS_bpf-fancy.o += -I$(objtree)/usr/include
 HOSTCFLAGS_bpf-fancy.o += -idirafter $(objtree)/include

samples/vfs/Makefile

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