sh: Do not use hyphen in exported variable names

sh: Do not use hyphen in exported variable names

arch/sh/Makefile defines and exports ld-bfd to be used by
arch/sh/boot/Makefile and arch/sh/boot/compressed/Makefile.  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 ld_bfd.

(Another instance of this problem was fixed upstream by commit
82977af93a0d "sh: rename suffix-y to suffix_y".)

References: https://buildd.debian.org/status/fetch.php?pkg=linux&arch=sh4&ver=4.13%7Erc5-1%7Eexp1&stamp=1502943967&raw=0
Fixes: ef9b542fce00 ("sh: bzip2/lzma uImage support.")
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Gbp-Pq: Topic bugfix/sh
Gbp-Pq: Name sh-boot-do-not-use-hyphen-in-exported-variable-name.patch