keys: avoid duplicating recovery keys

Setting PRODUCT_EXTRA_RECOVERY_KEYS duplicates the default key and
makes it so we can't set an extra key earlier in the build

Change-Id: I239d9994d9ad40b1779877ea2e89b55bd02e4c00
Revert "Revert "ota: Generate recovery-from-boot.p file""

This reverts commit ad2bf21609a3aa60d30c1ca0812c55576baa71ef.
Merge https://github.com/CyanogenMod/android_build into HEAD