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>

common/cmd_itest.c

@@ -191,5 +191,5 @@ int do_itest ( cmd_tbl_t *cmdtp, int flag, int argc, char *argv[] )
 U_BOOT_CMD(
 	itest, 4, 0, do_itest,
 	"return true/false on integer compare",
-	"[.b, .w, .l, .s] [*]value1 <op> [*]value2\n"
+	"[.b, .w, .l, .s] [*]value1 <op> [*]value2"
 );