Squash commits for public release

boot/riscv64/prekernel/drivers/uart.c

@@ -0,0 +1,24 @@
+#include "uart.h"
+
+volatile uint32_t* output = NULL;
+
+#define UART 0x10000000
+#define UART_THR (char*)(UART + 0x00) // THR:transmitter holding register
+#define UART_LSR (char*)(UART + 0x05) // LSR:line status register
+#define UART_LSR_EMPTY_MASK 0x40 // LSR Bit 6: Transmitter empty; both the THR and LSR are empty
+
+void uart_init()
+{
+    output = (uint32_t*)UART;
+}
+
+int uart_write(uint8_t data)
+{
+    if (!output) {
+        return 1;
+    }
+
+    while ((*UART_LSR & UART_LSR_EMPTY_MASK) == 0) { }
+    *output = data;
+    return 0;
+}