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/ads5121/ads5121_diu.c b/board/ads5121/ads5121_diu.c
index 41a1353..1dd60af 100644
--- a/board/ads5121/ads5121_diu.c
+++ b/board/ads5121/ads5121_diu.c
@@ -131,7 +131,7 @@
 	diufb, CONFIG_SYS_MAXARGS, 1, ads5121diu_init_show_bmp,
 	"Init or Display BMP file",
 	"init\n    - initialize DIU\n"
-	"addr\n    - display bmp at address 'addr'\n"
+	"addr\n    - display bmp at address 'addr'"
 	);