i believe i found the answer: config PHYS_OFFSET in my device kernel code.
[kernel code dir]/arch/arm/Kconfig:

config PHYS_OFFSET
        hex "Physical address of main memory" if MMU
        depends on !ARM_PATCH_PHYS_VIRT
        default DRAM_BASE if !MMU
        default 0x00000000 if ARCH_EBSA110 || \
                        ARCH_FOOTBRIDGE || \
                        ARCH_INTEGRATOR || \
                        ARCH_IOP13XX || \
                        ARCH_KS8695 || \
                        ARCH_REALVIEW
        default 0x10000000 if ARCH_OMAP1 || ARCH_RPC
        default 0x20000000 if ARCH_S5PV210
        default 0xc0000000 if ARCH_SA1100
        help
          Please provide the physical address corresponding to the
          location of main memory in your system.

The UBports docs describes the process of finding the offset base of boot.img really briefly, and its not very helpful. Is there a script to find it? or where to search for the "devices that share the same SoC" base offset's?