mtd/uclinux: Use generic __bss_stop instead of _ebss

The standard (see BSS_SECTION() in <asm-generic/vmlinux.lds.h> and
<asm-generic/sections.h>) symbol for the end of BSS is __bss_stop.
This allows to remove all local declarations that have been added to
several architectures just to please CONFIG_MTD_UCLINUX.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Paul Mundt <lethal@linux-sh.org>
Acked-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Michal Simek <monstr@monstr.eu>
Acked-by: Greg Ungerer <gerg@uclinux.org>