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/ppc-softmmu.mak b/default-configs/ppc-softmmu.mak
index d075e89..8b6be33 100644
--- a/default-configs/ppc-softmmu.mak
+++ b/default-configs/ppc-softmmu.mak
@@ -6,6 +6,7 @@
 CONFIG_ESCC=y
 CONFIG_M48T59=y
 CONFIG_VGA_PCI=y
+CONFIG_SERIAL=y
 CONFIG_I8254=y
 CONFIG_FDC=y
 CONFIG_IDE_QDEV=y