kbuild: Do not use hyphen in exported variable name

This definition in Makefile.dtbinst:

    export dtbinst-root ?= $(obj)

should define and export dtbinst-root when handling the root dts
directory, and do nothing in the subdirectories.  However some shells,
including dash, will not pass through environment variables whose name
includes a hyphen.  Usually GNU make does not use a shell to recurse,
but if e.g. $(srctree) contains '~' it will use a shell here.

Rename the variable to dtbinst_root.

References: https://bugs.debian.org/833561
Fixes: 323a028d39cdi ("dts, kbuild: Implement support for dtb vendor subdirs")
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/Makefile.dtbinst

@@ -14,7 +14,7 @@ src := $(obj)
 PHONY := __dtbs_install
 __dtbs_install:
 
-export dtbinst-root ?= $(obj)
+export dtbinst_root ?= $(obj)
 
 include include/config/auto.conf
 include scripts/Kbuild.include
@@ -27,7 +27,7 @@ dtbinst-dirs	:= $(dts-dirs)
 quiet_cmd_dtb_install =	INSTALL $<
       cmd_dtb_install =	mkdir -p $(2); cp $< $(2)
 
-install-dir = $(patsubst $(dtbinst-root)%,$(INSTALL_DTBS_PATH)%,$(obj))
+install-dir = $(patsubst $(dtbinst_root)%,$(INSTALL_DTBS_PATH)%,$(obj))
 
 $(dtbinst-files): %.dtb: $(obj)/%.dtb
 	$(call cmd,dtb_install,$(install-dir))