Kconfig: Fix CONFIG_GDB_STUB dependencies

If we select CONFIG_GDB_STUB without CONFIG_SERIAL:

build/console/console.romstage.o: In function `__gdb_hw_init':
[...]src/include/console/uart.h:74: undefined reference to `uart_init'
build/console/console.romstage.o: In function `__gdb_tx_byte':
[...]/src/include/console/uart.h:75: undefined reference to `uart_tx_byte'
build/console/console.romstage.o: In function `__gdb_tx_flush':
[...]/src/include/console/uart.h:76: undefined reference to `uart_tx_flush'
build/console/console.romstage.o: In function `__gdb_rx_byte':
[...]/src/include/console/uart.h:77: undefined reference to `uart_rx_byte'

Note that CONFIG_GDB_STUB should also work trough usbdebug,
But due to the lack of testing, it has been disabled when added.
This commit gives more information on the issue:
f2f7f03 console: Add console for GDB

Change-Id: I9accf8189dfd2c4ae379c03649d2e5863183457b
Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@no-log.org>
Reviewed-on: https://review.coreboot.org/12708
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>
diff --git a/src/Kconfig b/src/Kconfig
index 46349c5..6fbc1f0 100644
--- a/src/Kconfig
+++ b/src/Kconfig
@@ -728,6 +728,7 @@
 config GDB_STUB
 	bool "GDB debugging support"
 	default n
+	depends on CONSOLE_SERIAL
 	help
 	  If enabled, you will be able to set breakpoints for gdb debugging.
 	  See src/arch/x86/lib/c_start.S for details.