How to stop booting in U-Boot and get U-Boot command line?

I tried both Debian and Ubuntu images for ROCK Pi N10 and in both cases I can’t stop booting in U-Boot to modify the booting process (e.g. to add my DT overlay).
When I renamed the boot.scr in /boot partition, the board tries to boot from /boot/extlinux directory.
When I rename the /boot/extlinux, it goes to U-Boot command line, but it can’t read from serial console (even though it writes to it!).
How can I cure that problem?
The minicom settings are correct, as when Linux boots normally, I can communicate via serial console in both directions.

Thank you in advance,
Best regards,

I can see, that the kernel uses the overlay: rk3399pro-console-on-uart2
Does it mean that I need to modify the U-Boot DT in a similar way?

For the question of stopping booting in U-Boot. So far the U-Boot does not support this.

In all other single board computers I used before it is normal that I can enter the U-Boot command line.
Why it is not possible in Rockpi N10?

Well, I have found the workaround. I simply modify the boot.scr (and hence boot.cmd) in /boot so that it first tries to load myboot.cmd from my TFTP server. If it succeeds, it executes it. If not, it executes the unmodified part of the boot.scr (boot.cmd).
That’s not as convenient as interactive work with u-boot CLI but at least allows to avoid continuous reflashing, when I have to modify DT & kernel which often leads to non-booting configurations.

Hi. After checking the schematic, ROCK Pi N10 uses UART2 Channel C for debug console by default.
if you want to use U-Boot console, try to add this patch for U-Boot.

 36 diff --git a/arch/arm/mach-rockchip/rk3399/rk3399.c b/arch/arm/mach-rockchip/rk3399/rk3399.c
 37 index ba5f731..f658a39 100644
 38 --- a/arch/arm/mach-rockchip/rk3399/rk3399.c
 39 +++ b/arch/arm/mach-rockchip/rk3399/rk3399.c
 40 @@ -140,16 +140,16 @@ void board_debug_uart_init(void)
 41     rk_clrsetreg(&grf->gpio4c_iomux,
 42              GRF_GPIO4C4_SEL_MASK,
 43              GRF_UART2DBGC_SOUT << GRF_GPIO4C4_SEL_SHIFT);
 44 -#if defined(CONFIG_ROCKCHIP_RK3399PRO)
 45 -   /* Set channel A as UART2 input */
 46 -   rk_clrsetreg(&grf->soc_con7,
 47 -            GRF_UART_DBG_SEL_MASK,
 49 -#else
 50 +//#if defined(CONFIG_ROCKCHIP_RK3399PRO)
 51 +// /* Set channel A as UART2 input */
 52 +// rk_clrsetreg(&grf->soc_con7,
 53 +//          GRF_UART_DBG_SEL_MASK,
 55 +//#else
 56     /* Set channel C as UART2 input */
 57     rk_clrsetreg(&grf->soc_con7,
 58              GRF_UART_DBG_SEL_MASK,
 60 -#endif
 61 +//#endif
 62  #endif
 63  }

Yes, it helped! Thanks a lot.