init: make fatal reboot target configurable

Currently, if init encounters a fatal issues it reboots to fastboot
but this may be not desirable in all cases, especially the case of
critical services crashing.  Therefore this change adds the ability
for vendors to customize the reboot target via the
androidboot.init_fatal_reboot_target= kernel command line.

This applies to all LOG(FATAL) messages as well as fatal signals in
userdebug/eng builds, except for signals before logging is enabled in
first stage init.

Bug: 121006328
Test: device reboots to configurable target with LOG(FATAL)
Test: device reboots to configurable target after a segfault in the
      various stages of init
Test: device reboots to fastboot without a configured target
Change-Id: I16ea9e32e2fee08dece3d33b697d7a08191d607b
Merged-In: I16ea9e32e2fee08dece3d33b697d7a08191d607b
(cherry picked from commit 75e13baf)