hush: implement echo builtin

builtin_echo                                           -      36     +36
bltins                                               384     396     +12
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 1/0 up/down: 48/0)               Total: 48 bytes

diff --git a/shell/Config.in b/shell/Config.in
index 0689b4e..312583e 100644
--- a/shell/Config.in
+++ b/shell/Config.in
@@ -166,6 +166,7 @@
 	select TRUE
 	select FALSE
 	select TEST
+	select ECHO
 	help
 	  hush is a very small shell (just 18k) and it has fairly complete
 	  Bourne shell grammar.  It even handles all the normal flow control