hush: make echo builtin optional

It's a bit overkill (who would want it off?) but ash already has it
configurable. Let's be symmetric.

Also tweak kbuild logic to use ASH_BUILTIN_ECHO to select echo.o,
not ASH.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/coreutils/Kbuild.src b/coreutils/Kbuild.src
index 9ac6b9e..d0b3f62 100644
--- a/coreutils/Kbuild.src
+++ b/coreutils/Kbuild.src
@@ -14,15 +14,11 @@
 lib-$(CONFIG_LESS)          += cat.o   # less too
 lib-$(CONFIG_CRONTAB)       += cat.o   # crontab -l
 lib-$(CONFIG_ADDUSER)       += chown.o # used by adduser
-lib-$(CONFIG_ADDGROUP)      += chown.o # used by adduser
+lib-$(CONFIG_ADDGROUP)      += chown.o # used by addgroup
 lib-$(CONFIG_FTPD)          += ls.o    # used by ftpd
 
-lib-$(CONFIG_ASH)           += echo.o  # used by ash
-lib-$(CONFIG_SH_IS_ASH)     += echo.o  # used by ash
-lib-$(CONFIG_BASH_IS_ASH)   += echo.o  # used by ash
-lib-$(CONFIG_HUSH)          += echo.o  # used by hush
-lib-$(CONFIG_SH_IS_HUSH)    += echo.o  # used by hush
-lib-$(CONFIG_BASH_IS_HUSH)  += echo.o  # used by hush
+lib-$(CONFIG_ASH_BUILTIN_ECHO) += echo.o
+lib-$(CONFIG_HUSH_ECHO)     += echo.o
 
 lib-$(CONFIG_ASH_BUILTIN_PRINTF) += printf.o
-lib-$(CONFIG_HUSH_PRINTF) += printf.o
+lib-$(CONFIG_HUSH_PRINTF)   += printf.o