That will not work as bootloader is for sure interpreting them as well, it will also set them but it doesn't clear them.
Setting:
# Make sure that UART is enabled
fw_setenv boot_wait on
fw_setenv uart_en 1
# Enforce single partition.
fw_setenv flag_boot_rootfs 0
fw_setenv flag_last_success 0
fw_setenv flag_boot_success 1
fw_setenv flag_try_sys1_failed 8
fw_setenv flag_try_sys2_failed 8
Should really be bulletproof