Rename CONFIG_SYS_TEXT_BASE to CONFIG_TEXT_BASE

The current name is inconsistent with SPL which uses CONFIG_SPL_TEXT_BASE
and this makes it imposible to use CONFIG_VAL().

Rename it to resolve this problem.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/board/kontron/sl28/Kconfig b/board/kontron/sl28/Kconfig
index abcacc3..2981e57 100644
--- a/board/kontron/sl28/Kconfig
+++ b/board/kontron/sl28/Kconfig
@@ -12,7 +12,7 @@
 config SYS_CONFIG_NAME
 	default "kontron_sl28"
 
-config SYS_TEXT_BASE
+config TEXT_BASE
 	default 0x96000000
 
 config SL28_SPL_LOADS_ATF_BL31