mkimg: increase image size to fit 16GB storage device

* This will create a flashable image that is 15500000000 bytes.
* I checked several 16GB sdcards and USB storage devices and the
  actual byte size ranged from 15646851072 to 16008609792.

mkimg.sh

@@ -29,7 +29,7 @@ VERSION=RaspberryVanillaAOSP15
 DATE=$(date +%Y%m%d)
 TARGET=$(echo ${TARGET_PRODUCT} | sed 's/^aosp_//')
 IMGNAME=${VERSION}-${DATE}-${TARGET}.img
-IMGSIZE=7GiB
+IMGSIZE=15.5GB
 
 if [ -f ${ANDROID_PRODUCT_OUT}/${IMGNAME} ]; then
   exit_with_error "${ANDROID_PRODUCT_OUT}/${IMGNAME} already exists!"