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>