From mboxrd@z Thu Jan  1 00:00:00 1970
From: Sean Anderson <seanga2@gmail.com>
Date: Sat, 11 Jan 2020 15:42:33 -0500
Subject: How to debug HW startup?
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> Could You share a Linkit  _defconfig with early serial debug enabled?
> I'm decidedly missing something as, even enabling
> 
> CONFIG_DEBUG_UART=y
> CONFIG_DEBUG_UART_BASE=0x10000e00
> CONFIG_DEBUG_UART_CLOCK=20
> CONFIG_DEBUG_UART_SHIFT=2
> CONFIG_DEBUG_UART_ANNOUNCE=y
> 
> I still have plenty of errors:
> 
> /home/mcon/vocore/__V2__/Buildroot/recov/host/bin/mipsel-linux-ld.bfd:
> arch/mips/cpu/start.o: in function `wr_done':
> (.text+0x650): undefined reference to `debug_uart_init'
> /home/mcon/vocore/__V2__/Buildroot/recov/host/bin/mipsel-linux-ld.bfd:
> (.text+0x654): undefined reference to `debug_uart_init'
> /home/mcon/vocore/__V2__/Buildroot/recov/host/bin/mipsel-linux-ld.bfd:
> board/vocore/vocore2/built-in.o: in function `board_early_init_f':
> (.text.board_early_init_f+0x10): undefined reference to `printhex8'
> /home/mcon/vocore/__V2__/Buildroot/recov/host/bin/mipsel-linux-ld.bfd:
> common/built-in.o: in function `putc':
> (.text.putc+0x18): undefined reference to `printch'
> /home/mcon/vocore/__V2__/Buildroot/recov/host/bin/mipsel-linux-ld.bfd:
> common/built-in.o: in function `puts':
> (.text.puts+0x2c): undefined reference to `printch'
>

You need to pick a debug uart driver, e.g. CONFIG_DEBUG_UART_NS16550.