m68k: move CONFIG_SYS_TEXT_BASE to defconfig files

For historical reason, CONFIG_SYS_TEXT_BASE has been specified
in various ways:

[1] by board/${VENDOR}/${BOARD}/config.mk

[2] by CONFIG_SYS_EXTRA_OPTIONS
    (This was "options" field of boards.cfg before Kconfig conversion)

[3] by include/configs/${BOARD}.h

[4] by configs/${BOARD}_defconfig

Most of M68K boards use either [1] or [2], both of which we want to
deprecate.  Switch them into [4], which is the newest way (Kconfig).

We still allow [3] too, because it is still used by many boards and
we expect much time for conversion.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Angelo Dureghello <angelo at sysam.it>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/M53017EVB_defconfig b/configs/M53017EVB_defconfig
index ca538f4..e86fe5b 100644
--- a/configs/M53017EVB_defconfig
+++ b/configs/M53017EVB_defconfig
@@ -1,4 +1,5 @@
 CONFIG_M68K=y
 CONFIG_TARGET_M53017EVB=y
+CONFIG_SYS_TEXT_BASE=0x00000000
 CONFIG_SYS_PROMPT="-> "
 # CONFIG_CMD_SETEXPR is not set