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/cpu/nios2/sysid.c b/cpu/nios2/sysid.c
index 251204b..77bc8b5 100644
--- a/cpu/nios2/sysid.c
+++ b/cpu/nios2/sysid.c
@@ -53,6 +53,6 @@
 U_BOOT_CMD(
 	sysid,	1,	1,	do_sysid,
 	"display Nios-II system id",
-	"\n    - display Nios-II system id\n"
+	""
 );
 #endif /* CONFIG_SYS_NIOS_SYSID_BASE */