Provide an easier way to specify custom linker script.

Issue: Currently in the Makefile both DEVICE and LDSCRIPT cannot be specified at the same time. This makes it difficult to have different linker script while keep all the other flags same.

This is especially true for bootloader projects and/or projects that will have to work with a custom bootloader.

It will be nice to have a way to use a custom linker script while keeping all the other settings as is.