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/board/mpl/vcma9/cmd_vcma9.c b/board/mpl/vcma9/cmd_vcma9.c
index cbe7fe2..0160774 100644
--- a/board/mpl/vcma9/cmd_vcma9.c
+++ b/board/mpl/vcma9/cmd_vcma9.c
@@ -174,5 +174,5 @@
 U_BOOT_CMD(
 	vcma9, 6, 1, do_vcma9,
 	"VCMA9 specific commands",
-	"flash mem [SrcAddr]\n    - updates U-Boot with image in memory\n"
+	"flash mem [SrcAddr]\n    - updates U-Boot with image in memory"
 );