Have you seen FSF's summary on Single-Board Computers?
Basically, i.MX6 is the most free SoC at the moment, closely followed by some Allwiners, Rockchips, OMAPs, and the Ingenic used by MIPS Creator.
Those will boot without blobs, though some optional features like GPU or video decoding may not work without blobs.
All the rest - Broadcom VideoCore, Samsung Exynos, Quallcomm Snapdragon - don't give you control over the boot process.
I'm not sure about Tegra though.
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