FP2 Reboot Cycle

Any system modifications (root / openGapps / …)? Is the google app still enabled?
Solutions depend on your setup: e.g. if you did have openGapps installed, flash it again from recovery before booting (but after the update).

See also

P.S.
Some issues are caused by incompatibilities between user data and the upgrade process (e.g. the openGAPPs case). The nuclear option (i.e. destroy the data) to solve those is, when in fastboot mode (see manual upgrade instructions):
fastboot format userdata