[build] Build pcbios specific drivers only on pcbios

Signed-off-by: Piotr Jaroszyński <p.jaroszynski@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/Makefile

 SRCDIRS		+= arch/i386/core arch/i386/transitions arch/i386/prefix
 SRCDIRS		+= arch/i386/firmware/pcbios
 SRCDIRS		+= arch/i386/image
-SRCDIRS		+= arch/i386/drivers
-SRCDIRS		+= arch/i386/drivers/net
 SRCDIRS		+= arch/i386/interface/pcbios
 SRCDIRS		+= arch/i386/interface/pxe
 SRCDIRS		+= arch/i386/interface/pxeparent

src/arch/i386/Makefile.pcbios

 #
 LDFLAGS		+= -N --no-check-sections
 
+# pcbios specific drivers
+SRCDIRS		+= arch/i386/drivers
+SRCDIRS		+= arch/i386/drivers/net
+
 # Media types.
 #
 MEDIA		+= rom