add initial source and build system

.gitignore

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+/.rscons*
+/build/

Rsconscript

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+build do
+  Environment.new do |env|
+    env["CC"] = "x86_64-w64-mingw32-gcc"
+    env["CPPPATH"] += %w[/usr/include/efi /usr/include/efi/x86_64]
+    env["CFLAGS"] += %w[-ffreestanding]
+    env["LDFLAGS"] += %w[-nostdlib -Wl,-dll -shared -Wl,--subsystem,10 -e efi_main]
+    env["LIBS"] += %w[gcc]
+    env["sources"] = glob("*.c")
+    env.Program("^/BOOTX64.EFI", "${sources}")
+  end
+end

main.c

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+#include <efi.h>
+#include <efilib.h>
+
+EFI_STATUS
+efi_main(EFI_HANDLE image_handle, EFI_SYSTEM_TABLE * system_table)
+{
+    EFI_STATUS status;
+    EFI_INPUT_KEY key;
+
+    status = system_table->ConOut->OutputString(system_table->ConOut, L"My first EFI loader\n\r");
+    if (EFI_ERROR(status))
+        return status;
+
+    status = system_table->ConIn->Reset(system_table->ConIn, FALSE);
+    if (EFI_ERROR(status))
+        return status;
+
+    while ((status = system_table->ConIn->ReadKeyStroke(system_table->ConIn, &key)) == EFI_NOT_READY)
+    {
+    }
+
+    return status;
+}