thermosphere: add semihosting support & load a kernel using it when needed

basically host i/o

thermosphere/src/main.c

@@ -2,16 +2,50 @@
 #include "core_ctx.h"
 #include "debug_log.h"
 #include "platform/uart.h"
+#include "semihosting.h"
 #include "traps.h"
 
+extern const u8 __start__[];
+
+static void loadKernelViaSemihosting(void)
+{
+    size_t len = 1<<20; // max len
+    uintptr_t buf = (uintptr_t)__start__ + (1<<20);
+    long handle = -1, ret;
+
+    DEBUG("Loading kernel via semihosting file I/O... ");
+    handle = semihosting_file_open("test_kernel.bin", FOPEN_MODE_RB);
+    if (handle < 0) {
+        DEBUG("failed to open file (%ld)!\n", handle);
+        panic();
+    }
+
+    if ((ret = semihosting_file_read(handle, &len, buf)) < 0) {
+        DEBUG("failed to read file (%ld)!\n", ret);
+        panic();
+    }
+
+    DEBUG("OK!");
+    semihosting_file_close(handle);
+    currentCoreCtx->kernelEntrypoint = buf;
+}
+
 int main(void)
 {
     enableTraps();
 
-    if (currentCoreCtx->coreId == 0) {
+    if (currentCoreCtx->isColdBootCore) {
         uartInit(115200);
         DEBUG("Hello from Thermosphere!\n");
-        __builtin_trap();
+        if (currentCoreCtx->kernelEntrypoint == 0) {
+            if (semihosting_connection_supported()) {
+                loadKernelViaSemihosting();
+                //panic();
+            } else {
+                DEBUG("Kernel not loaded!\n");
+                panic();
+            }
+        }
     }
     else {
         DEBUG("Core %u booted\n", currentCoreCtx->coreId);