fastboot: Support flashing init_boot.img into init_boot partition

init_boot_a and init_boot_b partitions are used in A/B devices.

Bug: 203698939
Test: m

Change-Id: If00f581ab7ae340d78d7d55eebafd92ac15d2bc1
diff --git a/fastboot/fastboot.bash b/fastboot/fastboot.bash
index 5397455..e9bf9e9 100644
--- a/fastboot/fastboot.bash
+++ b/fastboot/fastboot.bash
@@ -109,7 +109,7 @@
 
     cur="${COMP_WORDS[COMP_CWORD]}"
     if [[ $i -eq $COMP_CWORD ]]; then
-        partitions="boot bootloader dtbo modem odm odm_dlkm oem product pvmfw radio recovery system system_dlkm vbmeta vendor vendor_dlkm"
+        partitions="boot bootloader dtbo init_boot modem odm odm_dlkm oem product pvmfw radio recovery system system_dlkm vbmeta vendor vendor_dlkm"
         COMPREPLY=( $(compgen -W "$partitions" -- $cur) )
     else
         _fastboot_util_complete_local_file "${cur}" '!*.img'