General help message cleanup

Many of the help messages were not really helpful; for example, many
commands that take no arguments would not print a correct synopsis
line, but "No additional help available." which is not exactly wrong,
but not helpful either.

Commit ``Make "usage" messages more helpful.'' changed this
partially. But it also became clear that lots of "Usage" and "Help"
messages (fields "usage" and "help" in struct cmd_tbl_s respective)
were actually redundant.

This patch cleans this up - for example:

Before:
	=> help dtt
	dtt - Digital Thermometer and Thermostat

	Usage:
	dtt         - Read temperature from digital thermometer and thermostat.

After:
	=> help dtt
	dtt - Read temperature from Digital Thermometer and Thermostat

	Usage:
	dtt

Signed-off-by: Wolfgang Denk <wd@denx.de>
diff --git a/board/delta/delta.c b/board/delta/delta.c
index 73e6c3e..84ff47e 100644
--- a/board/delta/delta.c
+++ b/board/delta/delta.c
@@ -260,7 +260,7 @@
 U_BOOT_CMD(
 	   kbd,	1,	1,	do_kbd,
 	   "read keyboard status",
-	   NULL
+	   ""
 );
 
 #endif /* DELTA_CHECK_KEYBD */