[BOOT] Add EFI headers and entry point

BOOT/ENVIRON/APP/BOOTMGR/bootmgr.c

@@ -0,0 +1,44 @@
+/*++
+
+Copyright (c) 2024, the LibreXP developers.
+Provided under the BSD 3-Clause license.
+
+Module Name:
+
+   bootmgr.c
+
+Abstract:
+
+   Main functions of the boot manager.
+
+--*/
+
+#include "bootmgr.h"
+
+NTSTATUS
+BmMain (
+    IN PBOOT_APPLICATION_PARAMETERS Parameters
+    )
+
+/*++
+
+Routine Description:
+
+    Firmware-independent boot manager entry point.
+
+Arguments:
+
+    Parameters - Input parameters for the boot manager.
+
+Return Value:
+
+    Error code on failure.
+    Does not return on success, as control is transferred to the OS loader.
+
+--*/
+
+{
+    /* TODO: Implement BmMain() */
+
+    return STATUS_SUCCESS;
+}