vboot_api_kernel.c: Remove kparams_ptr

Pass VbSelectAndLoadKernelParams kparams as a function argument instead
of using global variable kparams_ptr. Remove VbSelectAndLoadKernel and
replace its tests with the unit tests for vb2_set_boot_mode,
vb2api_kernel_phase2, vb2api_kernel_finalize, and vb2api_normal_boot.

BUG=b:172339016
BRANCH=none
TEST=make runtests

Cq-Depend: chromium:3731710
Signed-off-by: Hsin-Te Yuan <yuanhsinte@google.com>
Change-Id: I26895ced5e310b2894b9d42d0ad5514d3b0b930a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/3731412
Reviewed-by: Yu-Ping Wu <yupingso@chromium.org>
21 files changed