Allow installing extlinux.conf and boot.img with appended dtb at the same time

Currently, if user's deviceinfo contains i.e.:

deviceinfo_dtb="qcom/msm8916-longcheer-l8150"
deviceinfo_append_dtb="true"
deviceinfo_generate_bootimg="true"
deviceinfo_generate_extlinux_config="true"

The dtb will be appended to the kernel image and not copied, and extlinux aware bootloader won't be able to find the file.

This MR makes a series of changes:

• The dtbs are copied regardless of the append_dtb flag if some other boot method is also requested.
• The new deviceinfo property deviceinfo_extlinux_dtbdir is introduced to forcefully override fdtdir in extlinux.conf (i.e. to set it to /dtbs since pmOS usually ships all kernel's dts in /boot/dtbs)
• Use source_deviceinfo script in preparation for changes that would allow packages to add deviceinfo overrides in /etc/deviceinfo.d/*

Those changes are made to support future version of lk2nd that would make use of extlinux.conf specification in a similar to u-boot fashion, leveraging it's device database to pick the dtb blob. Allowing co-existence of boot.img and extlinux.conf is needed to aid in transition to the new config as well as to preserve the boot.img builds that are useful for debugging/broken boot recovery using fastboot.

Cc: @Minecrell