this error happens in sandbox with simple console, sandbox
has ARCH_HAS_CTRLC enabled by default

$ make sandbox_defconfig
$ make menuconfig
(select CONFIG_SHELL_SIMPLE)
$ make

LD      barebox
common/built-in.o: In function `__pr_memory_display':
/home/du/source/barebox/common/memory_display.c:117: undefined reference to `ctrlc'
common/built-in.o: In function `run_init':
/home/du/source/barebox/common/startup.c:317: undefined reference to `console_ctrlc_allow'
common/built-in.o: In function `parse_stream_outer':
/home/du/source/barebox/common/hush.c:1742: undefined reference to `ctrlc'
common/built-in.o: In function `run_list_real':
/home/du/source/barebox/common/hush.c:895: undefined reference to `ctrlc'
common/built-in.o: In function `run_shell':
/home/du/source/barebox/common/hush.c:1940: undefined reference to `ctrlc_handled'

Signed-off-by: DU HUANPENG <u74147@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>