[ARM] pxa: allow PXA to be built for multiple platforms

Convert the 'choice' into a 'menu' to allow multiple platforms to be
selected.

This means to do a build check across PXA, you don't end up spending
a lifetime building _twenty_ sodding kernels.
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>

arch/arm/mach-pxa/Kconfig

@@ -20,8 +20,7 @@ endmenu
 
 endif
 
-choice
-	prompt "Select target board"
+menu "Select target boards"
 
 config ARCH_GUMSTIX
 	bool "Gumstix XScale boards"
@@ -138,7 +137,7 @@ config MACH_PCM027
 	select PXA27x
 	select IWMMXT
 
-endchoice
+endmenu
 
 choice
 	prompt "Used baseboard"