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/common/cmd_dtt.c b/common/cmd_dtt.c
index 7783c88..3cfd36e 100644
--- a/common/cmd_dtt.c
+++ b/common/cmd_dtt.c
@@ -55,6 +55,6 @@
 
 U_BOOT_CMD(
 	  dtt,	1,	1,	do_dtt,
-	  "Digital Thermometer and Thermostat",
-	  "        - Read temperature from digital thermometer and thermostat.\n"
+	  "Read temperature from Digital Thermometer and Thermostat",
+	  ""
 );