efi_loader: correct file creation

The EFI shell expects that after opening a file with EFI_FILE_MODE_CREATE
GetInfo() succeeds. Up to now we do not actually create the file when
method Open() of the EFI_FILE_PROTOCOL is called.

If method Open() of the EFI_FILE_PROTOCOL is called with
EFI_FILE_MODE_CREATE and the file does not yet exist, call fs_write() with
a buffer size of zero to actually create the file.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>