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/pxa255_idp/pxa_idp.c b/board/pxa255_idp/pxa_idp.c
index aa598aa..e9e479c 100644
--- a/board/pxa255_idp/pxa_idp.c
+++ b/board/pxa255_idp/pxa_idp.c
@@ -129,7 +129,7 @@
 
 U_BOOT_CMD(idpcmd, CONFIG_SYS_MAXARGS, 0, do_idpcmd,
 	   "custom IDP command",
-	   "no args at this time\n"
+	   "no args at this time"
 );
 
 #endif