selftests/nolibc: add a "help" target

It presents the supported targets, and becomes the default target to
save the user from having to read the makefile. The "all" target was
placed after it and now points to "run" to do everything since it's
no longer the default one.
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

tools/testing/selftests/nolibc/Makefile

@@ -65,7 +65,32 @@ endif
 CFLAGS  ?= -Os -fno-ident -fno-asynchronous-unwind-tables
 LDFLAGS := -s
 
-all: nolibc-test
+help:
+	@echo "Supported targets under selftests/nolibc:"
+	@echo "  all          call the \"run\" target below"
+	@echo "  help         this help"
+	@echo "  sysroot      create the nolibc sysroot here (uses \$$ARCH)"
+	@echo "  nolibc-test  build the executable (uses \$$CC and \$$CROSS_COMPILE)"
+	@echo "  initramfs    prepare the initramfs with nolibc-test"
+	@echo "  defconfig    create a fresh new default config (uses \$$ARCH)"
+	@echo "  kernel       (re)build the kernel with the initramfs (uses \$$ARCH)"
+	@echo "  run          runs the kernel in QEMU after building it (uses \$$ARCH, \$$TEST)"
+	@echo "  rerun        runs a previously prebuilt kernel in QEMU (uses \$$ARCH, \$$TEST)"
+	@echo "  clean        clean the sysroot, initramfs, build and output files"
+	@echo ""
+	@echo "The output file is \"run.out\". Test ranges may be passed using \$$TEST."
+	@echo ""
+	@echo "Currently using the following variables:"
+	@echo "  ARCH          = $(ARCH)"
+	@echo "  CROSS_COMPILE = $(CROSS_COMPILE)"
+	@echo "  CC            = $(CC)"
+	@echo "  OUTPUT        = $(OUTPUT)"
+	@echo "  TEST          = $(TEST)"
+	@echo "  QEMU_ARCH     = $(if $(QEMU_ARCH),$(QEMU_ARCH),UNKNOWN_ARCH) [determined from \$$ARCH]"
+	@echo "  IMAGE_NAME    = $(if $(IMAGE_NAME),$(IMAGE_NAME),UNKNOWN_ARCH) [determined from \$$ARCH]"
+	@echo ""
+
+all: run
 
 sysroot: sysroot/$(ARCH)/include