If DTB-Id contains a subdirectory then as well as looking in the named location
also look in the parent directories.
This ensures that where DTBs have been moved by the kernel from the top level
into a subdirectory (as happened for arm64 in v3.19-rc1 when e.g.
apm-mustang.dtb moved to apm/apm-mustang.dtb) that flash-kernel can work with
both old and new kernels.
We search fully in /etc/flash-kernel/dtbs before looking in /usr/lib, so e.g.
we will find /etc/flash-kernel/dtbs/DTB before finding /usr/lib/.../DIR/DTB.
In v3.19-rc1 Linux arm64 moved all DTB files into per-vendor subdirectories and
the Debian kernel package has followed suite.
Update the entry for Mustang (the only arm64 platform in the current
flash-kernel database) to add the apm/ prefix and adjust the code which
installs and removes the dtb to cope with subdirectories in this context.