Makefile: Allow LTO to be disabled for a build

LTO (Link-Time Optimisation) is an very useful feature which can
significantly reduce the size of U-Boot binaries. So far it has been
made available for selected ARM boards and sandbox.

However, incremental builds are much slower when LTO is used. For example,
an incremental build of sandbox takes 2.1 seconds on my machine, but 6.7
seconds with LTO enabled.

Add a NO_LTO parameter to the build, similar to NO_SDL, so it can be
disabled during development if needed, for faster builds.

Add some documentation about LTO while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>

Makefile

@@ -643,6 +643,13 @@ export CFLAGS_EFI	# Compiler flags to add when building EFI app
 export CFLAGS_NON_EFI	# Compiler flags to remove when building EFI app
 export EFI_TARGET	# binutils target if EFI is natively supported
 
+export LTO_ENABLE
+
+# This is y if LTO is enabled for this build. See NO_LTO=1 to disable LTO
+ifeq ($(NO_LTO),)
+LTO_ENABLE=$(if $(CONFIG_LTO),y)
+endif
+
 # If board code explicitly specified LDSCRIPT or CONFIG_SYS_LDSCRIPT, use
 # that (or fail if absent).  Otherwise, search for a linker script in a
 # standard location.
@@ -708,16 +715,16 @@ endif
 LTO_CFLAGS :=
 LTO_FINAL_LDFLAGS :=
 export LTO_CFLAGS LTO_FINAL_LDFLAGS
-ifdef CONFIG_LTO
+ifeq ($(LTO_ENABLE),y)
 	ifeq ($(cc-name),clang)
-		LTO_CFLAGS		+= -flto
+		LTO_CFLAGS		+= -DLTO_ENABLE -flto
 		LTO_FINAL_LDFLAGS	+= -flto
 
 		AR			= $(shell $(CC) -print-prog-name=llvm-ar)
 		NM			= $(shell $(CC) -print-prog-name=llvm-nm)
 	else
 		NPROC			:= $(shell nproc 2>/dev/null || echo 1)
-		LTO_CFLAGS		+= -flto=$(NPROC)
+		LTO_CFLAGS		+= -DLTO_ENABLE -flto=$(NPROC)
 		LTO_FINAL_LDFLAGS	+= -fuse-linker-plugin -flto=$(NPROC)
 
 		# use plugin aware tools
@@ -1765,7 +1772,7 @@ ARCH_POSTLINK := $(wildcard $(srctree)/arch/$(ARCH)/Makefile.postlink)
 
 # Generate linker list symbols references to force compiler to not optimize
 # them away when compiling with LTO
-ifdef CONFIG_LTO
+ifeq ($(LTO_ENABLE),y)
 u-boot-keep-syms-lto := keep-syms-lto.o
 u-boot-keep-syms-lto_c := $(patsubst %.o,%.c,$(u-boot-keep-syms-lto))
 
@@ -1787,7 +1794,7 @@ endif
 
 # Rule to link u-boot
 # May be overridden by arch/$(ARCH)/config.mk
-ifdef CONFIG_LTO
+ifeq ($(LTO_ENABLE),y)
 quiet_cmd_u-boot__ ?= LTO     $@
       cmd_u-boot__ ?=								\
 		$(CC) -nostdlib -nostartfiles					\