allow ADK_TARGET_CPU for alpha

target/config/Config.in.cpu

@@ -3,6 +3,7 @@
 
 choice ADK_TARGET_CPU
 prompt "CPU" if ADK_TARGET_GENERIC || \
+		ADK_TARGET_SYSTEM_QEMU_ALPHA || \
 		ADK_TARGET_SYSTEM_QEMU_MIPS || \
 		ADK_TARGET_SYSTEM_QEMU_MIPS64 || \
 		ADK_TARGET_SYSTEM_QEMU_XTENSA