device/rockchip: add the 32bit support for rk3326 and px30

if you need build the 32bit by default, you need modify the xml on
.repo/manifests.
for example, the px30:
.repo/manifests$
--- a/px30_linux.xml
+++ b/px30_linux.xml
@@ -17,7 +17,7 @@
     <linkfile dest="mkfirmware.sh" src="common/mkfirmware.sh"/>
     <linkfile dest="build.sh" src="common/build.sh"/>
     <linkfile dest="rkflash.sh" src="common/rkflash.sh"/>
-    <linkfile dest="device/rockchip/.BoardConfig.mk" src="px30/BoardConfig.mk"/>
+    <linkfile dest="device/rockchip/.BoardConfig.mk" src="px30/BoardConfig_32bit.mk"/>
   </project>

Then run the build.sh

Change-Id: I86500f054d51bf185129dfa53fe9309252dcb6fb
Signed-off-by: Caesar Wang <wxt@rock-chips.com>

px30/BoardConfig.mk

@@ -13,7 +13,7 @@ export RK_BOOT_IMG=boot.img
 # parameter for GPT table
 export RK_PARAMETER=parameter-buildroot.txt
 # Buildroot config
-export RK_CFG_BUILDROOT=rockchip_px30
+export RK_CFG_BUILDROOT=rockchip_px30_64
 # Recovery config
 export RK_CFG_RECOVERY=rockchip_px30_recovery
 # Pcba config