efi_loader: log messages for bootefi command

Write log messages when booting via the bootefi command to allow tracking
on the syslog server. Example messages are

    Booting /snp.efi

or

    Booting /MemoryMapped(0x0,0x4fe00000,0x35a40)
    Loading image failed

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