efi_loader: EFI_PRINT instead of debug for variable services

For debug messages inside EFI API functions we should use the EFI_PRINT
macro which gives us well aligned output like:

EFI: Entry efi_get_variable("PlatformLang" ...)
  EFI: get 'efi_8be4df61-93ca-11d2-aa0d-00e098032b8c_PlatformLang'
EFI: Exit: efi_get_variable: 14

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