sbc8641d: increase monitor size from 256k to 384k

Between v2015.07-rc1 and v2015.07-rc2 this board started
silent boot failure.  A bisect led to commit 6eed3786
("net: Move the CMD_NET config to defconfigs").  This commit
looks harmless in itself, but it did implicitly add a feature
to the image which led to this:

 u-boot$git describe 6eed3786
 v2015.07-rc1-412-g6eed3786c68c
              ^^^

 u-boot$ls -l ../41*/u-boot.bin
 -rwxrwxr-x 1 paul paul 261476 Oct 16 16:47 ../411/u-boot.bin
 -rwxrwxr-x 1 paul paul 266392 Oct 16 16:43 ../412/u-boot.bin
 u-boot$bc
 bc 1.06.95
 Copyright 1991-1994, 1997, 1998, 2000, 2004, 2006 Free Software Foundation, Inc.
 This is free software with ABSOLUTELY NO WARRANTY.
 For details type `warranty'.
 256*1024
 262144

i.e. we finally broke through the 256k monitor size.  Jump it
up to 384k and fix the hard coded value used in the env offset
at the same time.

We were probably flirting with the 256k size issue without
knowing it when testing on different baselines in earlier
commits, but since this is all board specific, a rebase or
reorder to put this commit 1st is of little value.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>