makes: move common FLAGS to Makefile.common + rearrange makefiles

[nalajcie]

Description

• makes: move common FLAGS to Makefile.common
  Also:
  • introduce CPPFLAGS (C preprocessor flags)
  • export only target-related CFLAGS/LDFLAGS for ports
    (exceptions -f*-sections and --gc-sections)
  • remove unused RAM_SIZE define for armv7m
  • fix ports not being linked with --gc-sections
• makes: rename target-specific makefiles
  Use suffix (.mk) to represent the file type.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Motivation and Context

Reduce technology debt.

JIRA: CI-334

How Has This Been Tested?

• CI to pass: test PR for build changes phoenix-rtos-project#823

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing linter checks and tests passed.
• My changes generate no new compilation warnings for any of the targets.

Special treatment

• This PR needs additional PRs to work (list the PRs, preferably in merge-order).
• I will merge this PR by myself when appropriate.

[agkaminski]

LGTM