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/cpci5200/cpci5200.c b/board/esd/cpci5200/cpci5200.c
index b6b50fb..36fcf7f 100644
--- a/board/esd/cpci5200/cpci5200.c
+++ b/board/esd/cpci5200/cpci5200.c
@@ -295,4 +295,6 @@
 
 U_BOOT_CMD(writepci, 3, 1, do_writepci,
 	   "Write some data to pcibus",
-	   "<addr> <size>\n" "        - Write some data to pcibus.\n");
+	   "<addr> <size>\n"
+	   ""
+);