Fix init_boot partition AVB signing

Typo: 'BOARD_AVB_INIT_BOOT_IMAGE_KEY_PATH' ->
  'BOARD_AVB_INIT_BOOT_KEY_PATH'

Record avb_init_boot_* arguments to misc_info.txt

Add INIT_BOOT_SECURITY_PATCH to support per-partition SPL.

Bug: 203698939
Test: avbtool info_image --image init_boot.img
Change-Id: I63e082b1dd675138a6ab5cceb8814b3673d26b13