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/lwmon/lwmon.c b/board/lwmon/lwmon.c
index 1b665a9..75b3209 100644
--- a/board/lwmon/lwmon.c
+++ b/board/lwmon/lwmon.c
@@ -853,7 +853,7 @@
 	pic,	4,	1,	do_pic,
 	"read and write PIC registers",
 	"read  reg      - read PIC register `reg'\n"
-	"pic write reg val  - write value `val' to PIC register `reg'\n"
+	"pic write reg val  - write value `val' to PIC register `reg'"
 );
 
 /***********************************************************************
@@ -911,7 +911,7 @@
 U_BOOT_CMD(
 	kbd,	1,	1,	do_kbd,
 	"read keyboard status",
-	NULL
+	""
 );
 
 /* Read and set LSB switch */
@@ -985,7 +985,7 @@
 	"check and set LSB switch",
 	"on  - switch LSB on\n"
 	"lsb off - switch LSB off\n"
-	"lsb     - print current setting\n"
+	"lsb     - print current setting"
 );
 
 #endif