hw/avr: Add support for loading ELF/raw binaries

hw/avr: Add support for loading ELF/raw binaries

Add avr_load_firmware() function to load firmware in ELF or
raw binary format.

[AM: Corrected the type of the variable containing e_flags]
[AM: Moved definition of e_flags conversion function to boot.c]
Suggested-by: Aleksandar Markovic <aleksandar.m.mail@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Aleksandar Markovic <aleksandar.m.mail@gmail.com>
Reviewed-by: Aleksandar Markovic <aleksandar.m.mail@gmail.com>
Signed-off-by: Thomas Huth <huth@tuxfamily.org>
Message-Id: <20200705140315.260514-24-huth@tuxfamily.org>
[PMD: Replace load_image_targphys() by load_image_mr()]
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>