repack_bootimg: creates parent dirs for the copy-to files

When invoking repack_bootimg, it will copy files from the ramdisk
of src_bootimg into the ramdisk of dst_bootimg. However, if there
is no parent dirs then the copy will fail. Creating parent dirs
of the destination file if necessary.

Bug: 174443192
Test: repack_bootimg --src_bootimg boot-debug-4.19-gz.img --dst_bootimg boot-4.19-gz.img
Change-Id: I003954a92ca11b835591bfd4b82df40bf3d81381