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/common/cmd_misc.c b/common/cmd_misc.c
index 024299a..b97537e 100644
--- a/common/cmd_misc.c
+++ b/common/cmd_misc.c
@@ -56,7 +56,7 @@
 U_BOOT_CMD(
 	irqinfo,    1,    1,     do_irqinfo,
 	"print information about IRQs",
-	NULL
+	""
 );
 #endif
 
@@ -64,5 +64,5 @@
 	sleep ,    2,    1,     do_sleep,
 	"delay execution for some time",
 	"N\n"
-	"    - delay execution for N seconds (N is _decimal_ !!!)\n"
+	"    - delay execution for N seconds (N is _decimal_ !!!)"
 );