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/common/cmd_bootm.c b/common/cmd_bootm.c
index bd1813a..367d5a7 100644
--- a/common/cmd_bootm.c
+++ b/common/cmd_bootm.c
@@ -948,7 +948,7 @@
 	"\tbdt     - OS specific bd_t processing\n"
 	"\tcmdline - OS specific command line processing/setup\n"
 	"\tprep    - OS specific prep before relocation or go\n"
-	"\tgo      - start OS\n"
+	"\tgo      - start OS"
 );
 
 /*******************************************************************/
@@ -973,14 +973,14 @@
 U_BOOT_CMD(
 	boot,	1,	1,	do_bootd,
 	"boot default, i.e., run 'bootcmd'",
-	NULL
+	""
 );
 
 /* keep old command name "bootd" for backward compatibility */
 U_BOOT_CMD(
 	bootd, 1,	1,	do_bootd,
 	"boot default, i.e., run 'bootcmd'",
-	NULL
+	""
 );
 
 #endif
@@ -1068,7 +1068,7 @@
 	"addr [addr ...]\n"
 	"    - print header information for application image starting at\n"
 	"      address 'addr' in memory; this includes verification of the\n"
-	"      image contents (magic number, header and payload checksums)\n"
+	"      image contents (magic number, header and payload checksums)"
 );
 #endif
 
@@ -1135,7 +1135,7 @@
 	"list all images found in flash",
 	"\n"
 	"    - Prints information about all images found at sector\n"
-	"      boundaries in flash.\n"
+	"      boundaries in flash."
 );
 #endif