Hello Li yigang,
The question is not related to i.MX or linux BSP, just on how to run a application automatically when linux starts up.
Actually , you can get a lot of experience from internet.
for you question, it is very similar to boot on ubuntu :
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Open/home/root/.profile, and the line:
/usr/bin/chromium/chrome --no-sandbox --disable-setuid-sandbox --user-data-dir=/home/root
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Try it, please!
Have a nice day!
B.R,
weidong