kbuild: rename extmod-prefix to extmod_prefix

This seems to be useful in sub-make as well. As a preparation of
exporting it, rename extmod-prefix to extmod_prefix because exported
variables cannot contain hyphens.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

Makefile

@@ -919,7 +919,7 @@ endif
 ifdef CONFIG_LTO_CLANG
 ifdef CONFIG_LTO_CLANG_THIN
 CC_FLAGS_LTO	:= -flto=thin -fsplit-lto-unit
-KBUILD_LDFLAGS	+= --thinlto-cache-dir=$(extmod-prefix).thinlto-cache
+KBUILD_LDFLAGS	+= --thinlto-cache-dir=$(extmod_prefix).thinlto-cache
 else
 CC_FLAGS_LTO	:= -flto
 endif
@@ -1141,9 +1141,9 @@ endif # CONFIG_BPF
 
 PHONY += prepare0
 
-extmod-prefix = $(if $(KBUILD_EXTMOD),$(KBUILD_EXTMOD)/)
-export MODORDER := $(extmod-prefix)modules.order
-export MODULES_NSDEPS := $(extmod-prefix)modules.nsdeps
+extmod_prefix = $(if $(KBUILD_EXTMOD),$(KBUILD_EXTMOD)/)
+export MODORDER := $(extmod_prefix)modules.order
+export MODULES_NSDEPS := $(extmod_prefix)modules.nsdeps
 
 ifeq ($(KBUILD_EXTMOD),)
 core-y		+= kernel/ certs/ mm/ fs/ ipc/ security/ crypto/ block/
@@ -1742,7 +1742,7 @@ build-dirs := $(KBUILD_EXTMOD)
 $(MODORDER): descend
 	@:
 
-compile_commands.json: $(extmod-prefix)compile_commands.json
+compile_commands.json: $(extmod_prefix)compile_commands.json
 PHONY += compile_commands.json
 
 clean-dirs := $(KBUILD_EXTMOD)
@@ -1832,12 +1832,12 @@ endif
 
 PHONY += single_modpost
 single_modpost: $(single-no-ko) modules_prepare
-	$(Q){ $(foreach m, $(single-ko), echo $(extmod-prefix)$m;) } > $(MODORDER)
+	$(Q){ $(foreach m, $(single-ko), echo $(extmod_prefix)$m;) } > $(MODORDER)
 	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.modpost
 
 KBUILD_MODULES := 1
 
-export KBUILD_SINGLE_TARGETS := $(addprefix $(extmod-prefix), $(single-no-ko))
+export KBUILD_SINGLE_TARGETS := $(addprefix $(extmod_prefix), $(single-no-ko))
 
 # trim unrelated directories
 build-dirs := $(foreach d, $(build-dirs), \
@@ -1906,12 +1906,12 @@ nsdeps: modules
 quiet_cmd_gen_compile_commands = GEN     $@
       cmd_gen_compile_commands = $(PYTHON3) $< -a $(AR) -o $@ $(filter-out $<, $(real-prereqs))
 
-$(extmod-prefix)compile_commands.json: scripts/clang-tools/gen_compile_commands.py \
+$(extmod_prefix)compile_commands.json: scripts/clang-tools/gen_compile_commands.py \
 	$(if $(KBUILD_EXTMOD),,$(KBUILD_VMLINUX_OBJS) $(KBUILD_VMLINUX_LIBS)) \
 	$(if $(CONFIG_MODULES), $(MODORDER)) FORCE
 	$(call if_changed,gen_compile_commands)
 
-targets += $(extmod-prefix)compile_commands.json
+targets += $(extmod_prefix)compile_commands.json
 
 PHONY += clang-tidy clang-analyzer
 
@@ -1919,7 +1919,7 @@ ifdef CONFIG_CC_IS_CLANG
 quiet_cmd_clang_tools = CHECK   $<
       cmd_clang_tools = $(PYTHON3) $(srctree)/scripts/clang-tools/run-clang-tools.py $@ $<
 
-clang-tidy clang-analyzer: $(extmod-prefix)compile_commands.json
+clang-tidy clang-analyzer: $(extmod_prefix)compile_commands.json
 	$(call cmd,clang_tools)
 else
 clang-tidy clang-analyzer: