kbuild: rename KBUILD_ENABLE_EXTRA_GCC_CHECKS to KBUILD_EXTRA_WARN

KBUILD_ENABLE_EXTRA_GCC_CHECKS started as a switch to add extra warning
options for GCC, but now it is a historical misnomer since we use it
also for Clang, DTC, and even kernel-doc.

Rename it to more sensible, shorter KBUILD_EXTRA_WARN.

For the backward compatibility, KBUILD_ENABLE_EXTRA_GCC_CHECKS is still
supported (but not advertised in the documentation).

I also fixed up 'make help', and updated the documentation.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Sedat Dilek <sedat.dilek@gmail.com>
diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index 380a7d1..4a0cdd6 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -251,7 +251,7 @@
 DTC ?= $(objtree)/scripts/dtc/dtc
 
 # Disable noisy checks by default
-ifeq ($(findstring 1,$(KBUILD_ENABLE_EXTRA_GCC_CHECKS)),)
+ifeq ($(findstring 1,$(KBUILD_EXTRA_WARN)),)
 DTC_FLAGS += -Wno-unit_address_vs_reg \
 	-Wno-unit_address_format \
 	-Wno-avoid_unnecessary_addr_size \
@@ -262,7 +262,7 @@
 	-Wno-pci_device_reg
 endif
 
-ifneq ($(findstring 2,$(KBUILD_ENABLE_EXTRA_GCC_CHECKS)),)
+ifneq ($(findstring 2,$(KBUILD_EXTRA_WARN)),)
 DTC_FLAGS += -Wnode_name_chars_strict \
 	-Wproperty_name_chars_strict
 endif