flash_fp_mcu: Refactor to use main function
flash_fp_mcu: Refactor to use main function

https://google.github.io/styleguide/shellguide.html#s7.8-main

BRANCH=none
BUG=b:176500425
TEST=# dut1 is a dratini device
     scp util/flash_fp_mcu dut1:/usr/local/bin/flash_fp_mcu
     ssh dut1 flash_fp_mcu --hello; echo "Status $?"
     # Should succeed
     ssh dut1 flash_fp_mcu '/opt/google/biod/fw/bloonchipper*'; echo "Status $?"
     # Should succeed
     ssh dut1 flash_fp_mcu; echo "Status $?"
     # Should error out about no filename

Signed-off-by: Craig Hesling <383db090fd8ef907a606ac8a9eccd2962bc489cf@chromium.org>
Change-Id: I2f5f0c05e6c0a83a67948c5f914470b08a5d2c56
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2858276
Reviewed-by: Tom Hughes <fc5c12f30a5ed2baa0581f44ee14ad1eb7b20d83@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec