sh: Do not use hyphen in exported variable name

sh: Do not use hyphen in exported variable name

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

Other instances of this problem were previously fixed by commits
2bfbe7881ee0 "kbuild: Do not use hyphen in exported variable name"
and 82977af93a0d "sh: rename suffix-y to suffix_y".

Rename the variable to ld_bfd.

References: https://buildd.debian.org/status/fetch.php?pkg=linux&arch=sh4&ver=4.13%7Erc5-1%7Eexp1&stamp=1502943967&raw=0
Fixes: 7b022d07a0fd ("sh: Tidy up the ldscript output format specifier.")
Signed-off-by: Ben Hutchings <benh@debian.org>
Reviewed-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Signed-off-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>