modularization: boot component

Boot component prepares the very basic platform boot env. It finally call
into platform initilization entries:

- bsp_boot_init & cpu_secondary_init for start up
- or restore_s3_context for wakeup

this patch is the final one, it did some code clean up and move some definition
from vm0_boot.h to boot_context.h.

after this patch, the boot component include files:
arch/x86/boot/cpu_primary.S
arch/x86/boot/trampoline.S
arch/x86/boot/cpu_save_boot_ctx.S
arch/x86/boot/idt.S
boot/reloc.c
boot/include/reloc.h
include/arch/x86/boot/idt.h
include/arch/x86/boot/boot_context.h

Tracked-On: #1842
Signed-off-by: Jason Chen CJ <jason.cj.chen@intel.com>

hypervisor/include/arch/x86/guest/guest.h

@@ -220,7 +220,6 @@ int copy_from_gva(struct acrn_vcpu *vcpu, void *h_ptr, uint64_t gva,
  */
 int copy_to_gva(struct acrn_vcpu *vcpu, void *h_ptr, uint64_t gva,
 	uint32_t size, uint32_t *err_code, uint64_t *fault_addr);
-extern struct acrn_vcpu_regs vm0_boot_context;
 /**
  * @}
  */