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_spi.c

@@ -142,5 +142,5 @@ U_BOOT_CMD(
 	"<device> <bit_len> <dout> - Send <bit_len> bits from <dout> out the SPI\n"
 	"<device>  - Identifies the chip select of the device\n"
 	"<bit_len> - Number of bits to send (base 10)\n"
-	"<dout>    - Hexadecimal string that gets sent\n"
+	"<dout>    - Hexadecimal string that gets sent"
 );