powerpc/ieee1275: Read the db and dbx secure boot variables

Enhancing the infrastructure to enable the Platform Keystore (PKS) feature,
which provides access to the SB_VERSION, db, and dbx secure boot variables
from PKS.

If PKS is enabled, it will read secure boot variables such as db and dbx
from PKS and extract EFI Signature List (ESL) from it. The ESLs would be
saved in the Platform Keystore buffer, and the appendedsig module would
read it later to extract the certificate's details from ESL.

In the following scenarios, static key management mode will be activated:
 1. When Secure Boot is enabled with static key management mode
 2. When SB_VERSION is unavailable but Secure Boot is enabled
 3. When PKS support is unavailable but Secure Boot is enabled

Note:

 SB_VERSION: Key Management Mode
 1 - Enable dynamic key management mode. Read the db and dbx variables from PKS,
     and use them for signature verification.
 0 - Enable static key management mode. Read keys from the GRUB ELF Note and
     use it for signature verification.

Signed-off-by: Sudhakar Kuppusamy <sudhakar@linux.ibm.com>
Reviewed-by: Avnish Chouhan <avnish@linux.ibm.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/kern/ieee1275/ieee1275.c

@@ -23,7 +23,6 @@
 
 #define IEEE1275_PHANDLE_INVALID  ((grub_ieee1275_cell_t) -1)
 #define IEEE1275_IHANDLE_INVALID  ((grub_ieee1275_cell_t) 0)
-#define IEEE1275_CELL_INVALID     ((grub_ieee1275_cell_t) -1)