xtensa: use "test -e" instead of bashism "test -a"

On Ubuntu, /bin/sh is a symlink to dash, which does not support "test -a".
This causes messages like

    test: 1: -a: unexpected operator
    test: 1: -a: unexpected operator

and link failures like

    (.init.text+0x132): undefined reference to `platform_init'

due to the appropriate platform code not being compiled.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Chris Zankel <chris@zankel.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>