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>

cpu/arm_cortexa8/omap3/board.c

@@ -335,8 +335,8 @@ usage:
 U_BOOT_CMD(
 	nandecc, 2, 1,	do_switch_ecc,
 	"nandecc - switch OMAP3 NAND ECC calculation algorithm\n",
-	"[hw/sw] - Switch between NAND hardware (hw) or software (sw) ecc algorithm\n"
-	);
+	"[hw/sw] - Switch between NAND hardware (hw) or software (sw) ecc algorithm"
+);
 
 #endif /* CONFIG_NAND_OMAP_GPMC */