Control tests on RPi over UART

Select the 'ctrl_uart' hftest driver for Raspberry Pi 4. Implement
plat_console_get_char() to support reading the test arguments from
MiniUART.

Change-Id: I36901875a1f244638e9a6bad61f17d8977c9571b
diff --git a/BUILD.gn b/BUILD.gn
index 4a5acf0..f25e246 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -100,6 +100,7 @@
     gpio_base_address = "0xfe200000"
     aux_base_address = "0xfe215000"
     core_freq_mhz = 500
+    hftest_ctrl = "//test/hftest:ctrl_uart"
   }
 }