Here is another option that to me seems simpler and avoids patching sources and rebuilding.
Reconfigure your PADCTL_DEBUG_UART1_RX_0 (0x02435010) from HSIO/UARTA mode (0x00000458) to GPIO mode (0x00000059). This worked for me. I can press on the enter key all day long on the serial console and it won’t get through to U-Boot to stop the boot process. Then later when/if you need to use UART0/UARTA in say the kernel you can reconfigure it back to HSIO mode for UART support.
Here is the patch to disable it and make it a GPIO:
--- a/Linux_for_Tegra/bootloader/t186ref/BCT/tegra186-mb1-bct-pinmux-quill-p3310-1000-c03.cfg 2019-12-10 01:02:49.000000000 -0600
+++ b/Linux_for_Tegra/bootloader/t186ref/BCT/tegra186-mb1-bct-pinmux-quill-p3310-1000-c03.cfg 2020-01-24 10:58:07.025071028 -0600
@@ -299,7 +299,7 @@
pinmux.0x0243d008 = 0x00000402; # gpio_wan7_ph2: spi1, tristate-disable, input-disable, lpdr-disable
pinmux.0x0243d000 = 0x0000040a; # gpio_wan8_ph3: spi1, pull-up, tristate-disable, input-disable, lpdr-disable
pinmux.0x02435018 = 0x00000400; # uart1_tx_pt0: uarta, tristate-disable, input-disable, lpdr-disable
-pinmux.0x02435010 = 0x00000458; # uart1_rx_pt1: uarta, pull-up, tristate-enable, input-enable, lpdr-disable
+pinmux.0x02435010 = 0x00000059; # uart1_rx_pt1: rsvd1, pull-up, tristate-enable, input-enable, lpdr-disable
pinmux.0x0c302020 = 0x00000400; # uart3_tx_pw2: uartc, tristate-disable, input-disable, lpdr-disable
pinmux.0x0c302018 = 0x00000458; # uart3_rx_pw3: uartc, pull-up, tristate-enable, input-enable, lpdr-disable
pinmux.0x0c302010 = 0x00000400; # uart3_rts_pw4: uartc, tristate-disable, input-disable, lpdr-disable
Note that if you have a different board/module you will need to patch a different cfg file.
Following that change you can re-flash the pinmux settings using MB1_BCT:
sudo ./flash.sh -k MB1_BCT jetson-tx2 mmcblk0p1