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/evb64260/zuma_pbb.c b/board/evb64260/zuma_pbb.c
index aef9c27..cab4fca 100644
--- a/board/evb64260/zuma_pbb.c
+++ b/board/evb64260/zuma_pbb.c
@@ -204,19 +204,17 @@
 	zinit,	 1,	 0,	 do_zuma_init_pbb,
 	"init zuma pbb",
 	"\n"
-	"    - init zuma pbb\n"
 );
 U_BOOT_CMD(
 	zdtest,	  3,	  1,	  do_zuma_test_dma,
 	"run dma test",
 	"[cmd [count]]\n"
-	"    - run dma cmd (w=0,v=1,cp=2,cmp=3,wi=4,vi=5), count bytes\n"
+	"    - run dma cmd (w=0,v=1,cp=2,cmp=3,wi=4,vi=5), count bytes"
 );
 U_BOOT_CMD(
 	zminit,	  1,	  0,	  do_zuma_init_mbox,
 	"init zuma mbox",
 	"\n"
-	"    - init zuma mbox\n"
 );
 
 #endif