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/pf5200/pf5200.c b/board/esd/pf5200/pf5200.c
index f225f0e..4e64e83 100644
--- a/board/esd/pf5200/pf5200.c
+++ b/board/esd/pf5200/pf5200.c
@@ -294,7 +294,7 @@
 	return (0);
 }
 
-U_BOOT_CMD(poweroff, 1, 1, do_poweroff, "Switch off power", NULL);
+U_BOOT_CMD(poweroff, 1, 1, do_poweroff, "Switch off power", "");
 
 int phypower(int flag)
 {
@@ -338,7 +338,7 @@
 }
 
 U_BOOT_CMD(phypower, 2, 2, do_phypower,
-	   "Switch power of ethernet phy", NULL);
+	   "Switch power of ethernet phy", "");
 
 int do_writepci(cmd_tbl_t * cmdtp, int flag, int argc, char *argv[])
 {
@@ -369,5 +369,7 @@
 }
 
 U_BOOT_CMD(writepci, 3, 1, do_writepci,
-	   "Write some data to pcibus",
-	   "<addr> <size>\n" "        - Write some data to pcibus.\n");
+	"Write some data to pcibus",
+	"<addr> <size>\n"
+	""
+);