disk-label: add support for booting from GPT FAT partition
disk-label: add support for booting from GPT FAT partition

For a GPT+LVM combination disk, older bootloader that does not support
LVM, cannot load kernel from LVM.

The patch adds support to read from BASIC_DATA UUID partitions for the
case that the OS installer has installed the CHRP-BOOT config on a FAT
file system.

Makes GPT detection robust
* Fix bug in seek-pos updation code
* Check for Protective MBR Magic
* Check for valid GPT Signature
* Boundary check for allocated block size before reading into the
  buffer

Signed-off-by: Nikunj A Dadhania <e2f4a5db9b4735182a88051d1e7a3599f9eac9db@linux.vnet.ibm.com>
Reviewed-by: Thomas Huth <9e65be74847981e55b2564b6266c57d86d3bac05@redhat.com>
Signed-off-by: Alexey Kardashevskiy <ac1fc56530d7d037e1ea666f130daca22d72f964@ozlabs.ru>

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