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_i2c.c b/common/cmd_i2c.c
index b922b96..ae26845 100644
--- a/common/cmd_i2c.c
+++ b/common/cmd_i2c.c
@@ -1315,9 +1315,10 @@
 	"i2c crc32 chip address[.0, .1, .2] count - compute CRC32 checksum\n"
 	"i2c probe - show devices on the I2C bus\n"
 	"i2c reset - re-init the I2C Controller\n"
-	"i2c loop chip address[.0, .1, .2] [# of objects] - looping read of device\n"
+	"i2c loop chip address[.0, .1, .2] [# of objects] - looping read of device"
 #if defined(CONFIG_CMD_SDRAM)
-	"i2c sdram chip - print SDRAM configuration information\n"
+	"\n"
+	"i2c sdram chip - print SDRAM configuration information"
 #endif
 );