efi_loader: Clean up file size calculations

We recently added a common function for calculating file size,
instead of copy pasting the code around.  Switch one of the
occurences over to the common function

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviwed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>