kbuild: Do not use hyphen in exported variable name

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, the
variable does not reliably get exported to the environment, perhaps
because its name contains a hyphen.

Rename the variable to dtbinst_root.

References: https://bugs.debian.org/833561
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Gbp-Pq: Topic bugfix/all
Gbp-Pq: Name kbuild-do-not-use-hyphen-in-exported-variable-name.patch