Rename TEXT_BASE into CONFIG_SYS_TEXT_BASE

The change is currently needed to be able to remove the board
configuration scripting from the top level Makefile and replace it by
a simple, table driven script.

Moving this configuration setting into the "CONFIG_*" name space is
also desirable because it is needed if we ever should move forward to
a Kconfig driven configuration system.

Signed-off-by: Wolfgang Denk <wd@denx.de>
diff --git a/board/shannon/config.mk b/board/shannon/config.mk
index ca45733..6afa2d3 100644
--- a/board/shannon/config.mk
+++ b/board/shannon/config.mk
@@ -20,4 +20,4 @@
 #
 
 
-TEXT_BASE = 0xd8380000
+CONFIG_SYS_TEXT_BASE = 0xd8380000