Revert "kbuild: unexport sub_make_done to fix child make invocations"

Revert "kbuild: unexport sub_make_done to fix child make invocations"

This unfortunately introduces failure to build in other cases:
$ make ARCH=riscv CROSS_COMPILE=riscv64-unknown-linux-gnu- O=build \
	microchip_mpfs_generic_defconfig
  GEN     Makefile
#
# configuration written to .config
#
$ make ARCH=riscv CROSS_COMPILE=riscv64-unknown-linux-gnu- O=build
  HOSTCC  scripts/basic/fixdep
  GEN     Makefile
  HOSTCC  scripts/kconfig/conf.o
  YACC    scripts/kconfig/zconf.tab.[ch]
  LEX     scripts/kconfig/zconf.lex.c
  HOSTCC  scripts/kconfig/zconf.tab.o
  HOSTLD  scripts/kconfig/conf
scripts/kconfig/conf  --syncconfig Kconfig
***
*** Configuration file ".config" not found!
***
*** Please run some configurator (e.g. "make oldconfig" or
*** "make menuconfig" or "make xconfig").
***
make[4]: *** [/stuff/u-boot/scripts/kconfig/Makefile:75: syncconfig] Error 1
make[3]: *** [/stuff/u-boot/Makefile:702: syncconfig] Error 2
make[2]: *** [../Makefile:189: __sub-make] Error 2
make[1]: *** No rule to make target 'include/config/auto.conf', needed by 'include/config/uboot.release'.  Stop.
make: *** [Makefile:189: __sub-make] Error 2

This reverts commit 4284306d22c5b6d64ecd62b462551d9d313c8104.

Reported-by: Conor Dooley <conor@kernel.org>
Signed-off-by: Tom Rini <trini@konsulko.com>