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/g2000/g2000.c b/board/g2000/g2000.c
index b35038d..218f1be 100644
--- a/board/g2000/g2000.c
+++ b/board/g2000/g2000.c
@@ -186,7 +186,7 @@
 U_BOOT_CMD(
 	dumpebc,	1,	1,	do_dumpebc,
 	"Dump all EBC registers",
-	NULL
+	""
 );
 
 
@@ -208,7 +208,7 @@
 U_BOOT_CMD(
 	dumpdcr,	1,	1,	do_dumpdcr,
 	"Dump all DCR registers",
-	NULL
+	""
 );
 
 
@@ -272,6 +272,6 @@
 U_BOOT_CMD(
 	dumpspr,	1,	1,	do_dumpspr,
 	"Dump all SPR registers",
-	NULL
+	""
 );
 #endif