[platform][pc] Add basic UART support and connect the console to it.

This is especially useful when using the "nographic" option of qemu like this:

qemu -kernel build-pc-x86/lk.bin -nographic

Define WITH_CGA_CONSOLE=1 to enable the CGA console instead.

platform/pc/platform_p.h

@@ -25,6 +25,7 @@
 
 void platform_init_interrupts(void);
 void platform_init_timer(void);
+void platform_init_uart(void);
 
 #endif