kbuild: add CONFIG_HEADERS_INSTALL and loosen the dependency of samples

Commit 5318321d36 ("samples: disable CONFIG_SAMPLES for UML") used
a big hammer to fix the build errors under the samples/ directory.
Only some samples actually include uapi headers from usr/include.

Introduce CONFIG_HEADERS_INSTALL since 'depends on HEADERS_INSTALL' is
clearer than 'depends on !UML'. If this option is enabled, uapi headers
are installed before starting directory descending.

I added 'depends on HEADERS_INSTALL' to per-sample CONFIG options.
This allows UML to compile some samples.

$ make ARCH=um allmodconfig samples/
  [ snip ]
  CC [M]  samples/configfs/configfs_sample.o
  CC [M]  samples/kfifo/bytestream-example.o
  CC [M]  samples/kfifo/dma-example.o
  CC [M]  samples/kfifo/inttype-example.o
  CC [M]  samples/kfifo/record-example.o
  CC [M]  samples/kobject/kobject-example.o
  CC [M]  samples/kobject/kset-example.o
  CC [M]  samples/trace_events/trace-events-sample.o
  CC [M]  samples/trace_printk/trace-printk.o
  AR      samples/vfio-mdev/built-in.a
  AR      samples/built-in.a

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

samples/Kconfig

@@ -1,7 +1,6 @@
 # SPDX-License-Identifier: GPL-2.0-only
 menuconfig SAMPLES
 	bool "Sample kernel code"
-	depends on !UML
 	help
 	  You can build and test sample kernel code here.
 
@@ -95,16 +94,24 @@ config SAMPLE_CONFIGFS
 
 config SAMPLE_CONNECTOR
 	tristate "Build connector sample -- loadable modules only"
-	depends on CONNECTOR && m
+	depends on CONNECTOR && HEADERS_INSTALL && m
 	help
 	  When enabled, this builds both a sample kernel module for
 	  the connector interface and a user space tool to communicate
 	  with it.
 	  See also Documentation/connector/connector.txt
 
+config SAMPLE_HIDRAW
+	bool "hidraw sample"
+	depends on HEADERS_INSTALL
+
+config SAMPLE_PIDFD
+	bool "pidfd sample"
+	depends on HEADERS_INSTALL
+
 config SAMPLE_SECCOMP
 	bool "Build seccomp sample code"
-	depends on SECCOMP_FILTER
+	depends on SECCOMP_FILTER && HEADERS_INSTALL
 	help
 	  Build samples of seccomp filters using various methods of
 	  BPF filter construction.
@@ -156,6 +163,7 @@ config SAMPLE_ANDROID_BINDERFS
 
 config SAMPLE_VFS
 	bool "Build example programs that use new VFS system calls"
+	depends on HEADERS_INSTALL
 	help
 	  Build example userspace programs that use new VFS system calls such
 	  as mount API and statx().  Note that this is restricted to the x86

samples/Makefile

@@ -4,14 +4,14 @@
 obj-$(CONFIG_SAMPLE_ANDROID_BINDERFS)	+= binderfs/
 obj-$(CONFIG_SAMPLE_CONFIGFS)		+= configfs/
 obj-$(CONFIG_SAMPLE_CONNECTOR)		+= connector/
-subdir-y				+= hidraw
+subdir-$(CONFIG_SAMPLE_HIDRAW)		+= 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
+subdir-$(CONFIG_SAMPLE_PIDFD)		+= pidfd
 obj-$(CONFIG_SAMPLE_QMI_CLIENT)		+= qmi/
 obj-$(CONFIG_SAMPLE_RPMSG_CLIENT)	+= rpmsg/
 subdir-$(CONFIG_SAMPLE_SECCOMP)		+= seccomp