Use the linker script generator module

[BuFran]

Issues:

1. There is one rule missing in the gcc-rules.mk so the second commit must be edited after addition of this rule to the gcc-rules.mk
2. There must be added option SILENT to the gcc-rules.mk to be the build messages optionally more silent.
3. There must be done a way to conditionally delete the generated linker script during CLEAN process.

This pull request can go to master only after these issues will be corrected and some commits edited. But all features is working and ready for consideration.

[BuFran]

:red_circle: Until the PullRequest libopencm3/libopencm3#262 will be in master.