lib/coreboot_table: Add CBMEM_ID_VBOOT_WORKBUF pointer to coreboot table

Since struct vb2_shared_data already contains workbuf_size and
vboot_workbuf_size is never used in depthcharge, remove it from struct
sysinfo_t. In addition, remove lb_vboot_workbuf() and add
CBMEM_ID_VBOOT_WORKBUF pointer to coreboot table with
add_cbmem_pointers(). Parsing of coreboot table in libpayload is
modified accordingly.

BRANCH=none
BUG=chromium:1021452
TEST=emerge-nami coreboot libpayload depthcharge; Akali booted correctly

Change-Id: I890df3ff93fa44ed6d3f9ad05f9c6e49780a8ecb
Signed-off-by: Yu-Ping Wu <yupingso@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/37234
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Joel Kitching <kitching@google.com>
Reviewed-by: Julius Werner <jwerner@chromium.org>

src/security/vboot/misc.h

@@ -25,7 +25,6 @@ struct vb2_shared_data;
 /*
  * Source: security/vboot/common.c
  */
-void *vboot_get_workbuf(void);
 struct vb2_context *vboot_get_context(void);
 
 /*