build: allow to disable recovery-two-step.img generation

Android 7.1.2 introduced new mechanism for recovery updating in two-step OTAs.
Although, recovery-two-step.img generation may be completely broken on some devices with custom mkbootimg handling.
We can use recovery.img as base for that devices, so recovery-two-step.img generation will not be required in this case.
Let's add new TARGET_NO_TWO_STEP_RECOVERY flag to skip recovery-two-step.img generation during target files packaging if we need it.

Change-Id: Icbea2da2f9565277622746545cdb96bf7f5ef3df