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>

arch/xtensa/Makefile

@@ -68,8 +68,8 @@ endif
 
 # Only build variant and/or platform if it includes a Makefile
 
-buildvar := $(shell test -a $(srctree)/arch/xtensa/variants/$(VARIANT)/Makefile && echo arch/xtensa/variants/$(VARIANT)/)
-buildplf := $(shell test -a $(srctree)/arch/xtensa/platforms/$(PLATFORM)/Makefile && echo arch/xtensa/platforms/$(PLATFORM)/)
+buildvar := $(shell test -e $(srctree)/arch/xtensa/variants/$(VARIANT)/Makefile && echo arch/xtensa/variants/$(VARIANT)/)
+buildplf := $(shell test -e $(srctree)/arch/xtensa/platforms/$(PLATFORM)/Makefile && echo arch/xtensa/platforms/$(PLATFORM)/)
 
 # Find libgcc.a