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_usb.c b/common/cmd_usb.c
index a18e16e..7b8ee6b 100644
--- a/common/cmd_usb.c
+++ b/common/cmd_usb.c
@@ -686,14 +686,14 @@
 	"usb part [dev] - print partition table of one or all USB storage"
 	" devices\n"
 	"usb read addr blk# cnt - read `cnt' blocks starting at block `blk#'\n"
-	"    to memory address `addr'\n"
+	"    to memory address `addr'"
 );
 
 
 U_BOOT_CMD(
 	usbboot,	3,	1,	do_usbboot,
 	"boot from USB device",
-	"loadAddr dev:part\n"
+	"loadAddr dev:part"
 );
 
 #else
@@ -702,6 +702,6 @@
 	"USB sub-system",
 	"reset - reset (rescan) USB controller\n"
 	"usb  tree  - show USB device tree\n"
-	"usb  info [dev] - show available USB devices\n"
+	"usb  info [dev] - show available USB devices"
 );
 #endif