fat-files: Fix bug with root-entries = 0 on certain FAT32 file systems
fat-files: Fix bug with root-entries = 0 on certain FAT32 file systems

The maximum number of root directory entries can be zero on
certain FAT32 file systems. In this case, we've got to read
the cluster number of the root directory from the extended BPB
instead. See also the following URL for details:
https://en.wikipedia.org/wiki/Design_of_the_FAT_file_system#BPB20_OFS_06h

Signed-off-by: Thomas Huth <9e65be74847981e55b2564b6266c57d86d3bac05@redhat.com>
Signed-off-by: Alexey Kardashevskiy <ac1fc56530d7d037e1ea666f130daca22d72f964@ozlabs.ru>

aik/SLOF,open-power-host-os/slof,open-power-host-os/slof,stefanberger/SLOF-tpm,aik/SLOF,stefanberger/SLOF-tpm,open-power-host-os/slof,qemu/SLOF,qemu/SLOF,open-power-host-os/slof,stefanberger/SLOF-tpm,aik/SLOF,qemu/SLOF