refactor: drop posix-uefi from bootloader

Replace the vendored POSIX-UEFI runtime with a small local EFI entry and header so the bootloader depends only on the UEFI interfaces it actually uses.

Made-with: Cursor

Bootloader/Source/uefi/efi_entry.c

@@ -0,0 +1,23 @@
+#include "uefi.h"
+
+efi_handle_t IM = NULL;
+efi_system_table_t* ST = NULL;
+efi_boot_services_t* BS = NULL;
+efi_runtime_services_t* RT = NULL;
+
+efi_status_t bootloader_main(void);
+
+void __chkstk(void) {}
+
+efi_status_t EFIAPI efi_main(efi_handle_t image, efi_system_table_t* system_table) {
+    if (image == NULL || system_table == NULL || system_table->BootServices == NULL) {
+        return EFI_UNSUPPORTED;
+    }
+
+    IM = image;
+    ST = system_table;
+    BS = system_table->BootServices;
+    RT = system_table->RuntimeServices;
+
+    return bootloader_main();
+}