[kernel] Rename kernel main.cpp

Having main.cpp in the kernel and in the application being debugged is
annoying when setting breakpoints, so just like with main() vs
kernel_main(), kernel/main.cpp is now kernel/kernel_main.cpp.

src/kernel/kernel.module

@@ -26,8 +26,8 @@ kernel = module("kernel",
         "interrupts.cpp",
         "interrupts.s",
         "io.cpp",
+        "kernel_main.cpp",
         "logger.cpp",
-        "main.cpp",
         "memory.cpp",
         "memory.h.cog",
         "memory_bootstrap.cpp",