conditionalize Secure Boot initialization on x86 platform

conditionalize Secure Boot initialization on x86 platform

Orabug: 22364965

A previous commit, bbb87ba5690c72821614acdd465c24eeb99fa923,
introduced a call to efi_secure_boot_init() into the start_kernel()
sequence.  The new call was not conditionalized based on platform.

Secure Boot is only implmented on the x86 platform; the definition
of efi_secure_boot_init() is in arch/x86/platform/efi/efi.c.  On
non-x86 architectures, this caused a implicit-declaration error and
broke the build.

This commit wraps the call in #ifdef CONFIG_X86/#endif to eliminate
the error.

Signed-off-by: Dan Duval <dan.duval@oracle.com>
(cherry picked from commit 289ef170a213f9b078886327a162cbcb7a325838)
Signed-off-by: Dan Duval <dan.duval@oracle.com>