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/amcc/taishan/update.c b/board/amcc/taishan/update.c
index 63f8744..aab3a65 100644
--- a/board/amcc/taishan/update.c
+++ b/board/amcc/taishan/update.c
@@ -74,5 +74,5 @@
 }
 
 U_BOOT_CMD(update_boot_eeprom, 1, 1, do_update_boot_eeprom,
-	   "update bootstrap eeprom content", NULL);
+	   "update bootstrap eeprom content", "");
 #endif