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/doc/board/intel/slimbootloader.rst b/doc/board/intel/slimbootloader.rst
index 18f1cc0..87d71a5 100644
--- a/doc/board/intel/slimbootloader.rst
+++ b/doc/board/intel/slimbootloader.rst
@@ -69,7 +69,7 @@
     +GEN_CFG_DATA.PayloadId                     | 'U-BT'
 
 2. Update payload text base. PAYLOAD_EXE_BASE must be the same as U-Boot
-   CONFIG_SYS_TEXT_BASE in board/intel/slimbootloader/Kconfig.
+   CONFIG_TEXT_BASE in board/intel/slimbootloader/Kconfig.
    PAYLOAD_LOAD_HIGH must be 0::
 
     $ vi Platform/QemuBoardPkg/BoardConfig.py
@@ -122,7 +122,7 @@
 
 2. Update payload text base.
 
-* PAYLOAD_EXE_BASE must be the same as U-Boot CONFIG_SYS_TEXT_BASE
+* PAYLOAD_EXE_BASE must be the same as U-Boot CONFIG_TEXT_BASE
   in board/intel/slimbootloader/Kconfig.
 * PAYLOAD_LOAD_HIGH must be 0::