Compile serial only once

Push TARGET_WORDS_BIGENDIAN dependency to board level.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
diff --git a/default-configs/arm-softmmu.mak b/default-configs/arm-softmmu.mak
index 4c0fe22..02ad192 100644
--- a/default-configs/arm-softmmu.mak
+++ b/default-configs/arm-softmmu.mak
@@ -5,6 +5,7 @@
 CONFIG_ISA_MMIO=y
 CONFIG_NAND=y
 CONFIG_ECC=y
+CONFIG_SERIAL=y
 CONFIG_PTIMER=y
 CONFIG_SD=y
 CONFIG_MAX7310=y