loader: Implement .hex file loader

loader: Implement .hex file loader

This patch adds Intel Hexadecimal Object File format support to the
generic loader device.  The file format specification is available here:
http://www.piclist.com/techref/fileext/hex/intel.htm

This file format is often used with microcontrollers such as the
micro:bit, Arduino, STM32, etc.  Users expect to be able to run .hex
files directly with without first converting them to ELF.  Most
micro:bit code is developed in web-based IDEs without direct user access
to binutils so it is important for QEMU to handle this file format
natively.

Signed-off-by: Su Hang <suhang16@mails.ucas.ac.cn>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20180814162739.11814-6-stefanha@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>