ash: optional printf builtin. +25 bytes if off, +35 if on.
by Cristian Ionescu-Idbohrn.

diff --git a/coreutils/Kbuild b/coreutils/Kbuild
index cb45439..a5a2d4c 100644
--- a/coreutils/Kbuild
+++ b/coreutils/Kbuild
@@ -54,6 +54,7 @@
 lib-$(CONFIG_OD)        += od.o
 lib-$(CONFIG_PRINTENV)  += printenv.o
 lib-$(CONFIG_PRINTF)    += printf.o
+lib-$(CONFIG_ASH_BUILTIN_PRINTF) += printf.o
 lib-$(CONFIG_PWD)       += pwd.o
 lib-$(CONFIG_READLINK)  += readlink.o
 lib-$(CONFIG_REALPATH)  += realpath.o