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Not quite. Weakness is later in the chain.

Secure second-stage loader (BL2) loads secure third-stage loader (BL31) from insecure flash storage, then verifies it by calculating SHA-256 hash of it and comparing that to the value stored in the same insecure flash.

The Xilinx Zynq has one time programmable eFuse registers for this sort of thing

How easy is it to change the input of SHA256 without changing the output? That strikes me as still very difficult (MD5 is basically worthless for this reason).

For anyone wondering: the hash is part of the boot ROM that is being authenticated