pmos_install_functions: Increase boot partition size to 200MB

Installing from android recovery is failing due to insufficient space on /boot on Xiaomi MIX 2S (polaris). Increase it to 200MB resolved this issue.

I guess the best solution is that pmbootstrap can expose the boot_size config to install_options file, and read it in pmos_install_functions. But I don't know how to do that.

Anyway, I think this can be a band-aid before the best solution appears.