tools/nolibc: add a help target to list supported targets

The "help" target simply presents the list of supported targets
and the current set of variables being used to build the sysroot.

Since the help in tools/ suggests to use "install", which is
supported by most tools while such a target is not really relevant
here, an "install" target was also added, redirecting to "help".
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

tools/include/nolibc/Makefile

@@ -31,6 +31,23 @@ all_files := ctype.h errno.h nolibc.h signal.h std.h stdio.h stdlib.h string.h \
 # install all headers needed to support a bare-metal compiler
 all: headers
 
+install: help
+
+help:
+	@echo "Supported targets under nolibc:"
+	@echo "  all                 call \"headers\""
+	@echo "  clean               clean the sysroot"
+	@echo "  headers             prepare a sysroot in tools/include/nolibc/sysroot"
+	@echo "  headers_standalone  like \"headers\", and also install kernel headers"
+	@echo "  help                this help"
+	@echo ""
+	@echo "These targets may also be called from tools as \"make nolibc_<target>\"."
+	@echo ""
+	@echo "Currently using the following variables:"
+	@echo "  ARCH    = $(ARCH)"
+	@echo "  OUTPUT  = $(OUTPUT)"
+	@echo ""
+
 # Note: when ARCH is "x86" we concatenate both x86_64 and i386
 headers:
 	$(Q)mkdir -p $(OUTPUT)sysroot