Store the value from TARGET_OTA_ASSERT_DEVICE to a file

This is needed so it can be read in when re-signing a package to
allow for the proper device asserts.

Change-Id: I8f3bb491bbbaa4d2b827a45ef83f12c6c83e6712
(cherry picked from commit 85ed89f90320bf6352c6b2b43878fadc5fa9fc41)
Conflicts:
	core/Makefile