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>

lib/Kconfig.debug

@@ -305,14 +305,23 @@ config DEBUG_FS
 
 	  If unsure, say N.
 
-config HEADERS_CHECK
-	bool "Run sanity checks on uapi headers when building 'all'"
+config HEADERS_INSTALL
+	bool "Install uapi headers to usr/include"
 	depends on !UML
 	help
-	  This option will extract the user-visible kernel headers when
-	  building the 'all' target, and will run basic sanity checks on them to
-	  ensure that exported files do not attempt to include files which
-	  were not exported, etc.
+	  This option will install uapi headers (headers exported to user-space)
+	  into the usr/include directory for use during the kernel build.
+	  This is unneeded for building the kernel itself, but needed for some
+	  user-space program samples. It is also needed by some features such
+	  as uapi header sanity checks.
+
+config HEADERS_CHECK
+	bool "Run sanity checks on uapi headers when building 'all'"
+	depends on HEADERS_INSTALL
+	help
+	  This option will run basic sanity checks on uapi headers when
+	  building the 'all' target, for example, ensure that they do not
+	  attempt to include files which were not exported, etc.
 
 	  If you're making modifications to header files which are
 	  relevant for userspace, say 'Y', and check the headers