mkimg: adjust flashable image size

* It's been reported that 14848MiB = 15569256448 bytes is still too
  large to fit some 16GB storage devices.
* 15360000000 bytes is dividable by 16384, 4096, 2048, 1024, 512, ...
* Fixes b3a1d84235.

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=14848MiB
+IMGSIZE=15360000000
 
 if [ -f ${ANDROID_PRODUCT_OUT}/${IMGNAME} ]; then
   exit_with_error "${ANDROID_PRODUCT_OUT}/${IMGNAME} already exists!"