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/esd/tasreg/tasreg.c b/board/esd/tasreg/tasreg.c
index 9602ee5..1844442 100644
--- a/board/esd/tasreg/tasreg.c
+++ b/board/esd/tasreg/tasreg.c
@@ -250,8 +250,8 @@
 U_BOOT_CMD(
 	iploop,	2,	1,	do_iploop,
 	"i2c probe loop <addr>",
-	NULL
-	);
+	""
+);
 
 /*
  */
@@ -303,8 +303,8 @@
 U_BOOT_CMD(
 	codec,	1,	1,	do_codec,
 	"Enable codec",
-	NULL
-	);
+	""
+);
 
 /*
  */
@@ -336,8 +336,8 @@
 U_BOOT_CMD(
 	saa,	5,	1,	do_saa,
 	"Write to SAA1064 <addr> <instr> <cntrl> <data>",
-	NULL
-	);
+	""
+);
 
 /*
  */
@@ -375,8 +375,8 @@
 U_BOOT_CMD(
 	iwrite,	6,	1,	do_iwrite,
 	"Write n bytes to I2C-device",
-	"addr cnt data0 ... datan\n"
-	);
+	"addr cnt data0 ... datan"
+);
 
 /*
  */
@@ -407,8 +407,8 @@
 U_BOOT_CMD(
 	iread,	3,	1,	do_iread,
 	"Read from I2C <addr> <cnt>",
-	NULL
-	);
+	""
+);
 
 /*
  */
@@ -445,6 +445,6 @@
 U_BOOT_CMD(
 	ireadl,	2,	1,	do_ireadl,
 	"Read-loop from I2C <addr>",
-	NULL
-	);
+	""
+);
 #endif