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/mipsel-softmmu.mak b/default-configs/mipsel-softmmu.mak
index cf4121c..7ff0f03 100644
--- a/default-configs/mipsel-softmmu.mak
+++ b/default-configs/mipsel-softmmu.mak
@@ -4,6 +4,7 @@
 CONFIG_ESP=y
 CONFIG_VGA_PCI=y
 CONFIG_VGA_ISA=y
+CONFIG_SERIAL=y
 CONFIG_PARALLEL=y
 CONFIG_I8254=y
 CONFIG_PCSPK=y