Defining CONFIG_SYS_TEXT_BASE in config.mk is very old style.

Create CONFIG_SYS_TEXT_BASE option in Kconfig, but let it
depend on CONFIG_SPARC because we do not want to disturb
the other architectures that still define CONFIG_SYS_TEXT_BASE
in their header files.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Daniel Hellstrom <daniel@gaisler.com>