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_fdt.c

@@ -840,5 +840,5 @@ U_BOOT_CMD(
 	"fdt chosen [<start> <end>]          - Add/update the /chosen branch in the tree\n"
 	"                                        <start>/<end> - initrd start/end addr\n"
 	"NOTE: Dereference aliases by omiting the leading '/', "
-		"e.g. fdt print ethernet0.\n"
+		"e.g. fdt print ethernet0."
 );