'export suffix-y' does not work reliably because hyphens are disallowed
in shell variables.

A similar issue was fixed by commit 2bfbe788 ("kbuild: Do not use
hyphen in exported variable name").

If I do similar in dash, ARCH=sh fails to build.

  $ mv linux linux~
  $ cd linux~
  $ dash
  $ make O=foo/bar ARCH=sh CROSS_COMPILE=sh4-linux-gnu- defconfig all
  make[1]: Entering directory '/home/masahiro/linux~/foo/bar'
    [ snip ]
  make[4]: *** No rule to make target 'arch/sh/boot/compressed/vmlinux.bin.', needed by 'arch/sh/boot/compressed/piggy.o'.  Stop.
  make[3]: *** [/home/masahiro/linux~/arch/sh/boot/Makefile:40: arch/sh/boot/compressed/vmlinux] Error 2
  make[2]: *** [/home/masahiro/linux~/arch/sh/Makefile:194: zImage] Error 2
  make[1]: *** [/home/masahiro/linux~/Makefile:350: __build_one_by_one] Error 2
  make[1]: Leaving directory '/home/masahiro/linux~/foo/bar'
  make: *** [Makefile:219: __sub-make] Error 2

The maintainer of GNU Make stated that there is no consistent way to
export variables that do not meet the shell's naming criteria.
(https://savannah.gnu.org/bugs/?55719)

Consequently, you cannot use hyphens in exported variables.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <n.schier@avm.de>