build: Add menuselect options to facilitate code tracing and coverage

build: Add menuselect options to facilitate code tracing and coverage

The following options have been added to the menuselect "Compiler Flags"
section...

CODE_COVERAGE: The ability to enable code coverage via the `--enable-coverage`
configure flag has existed for many years but changing it requires
re-running ./configure which is painfully slow.  With this commit, you can
now enable and disable it via menuselect. Setting this option adds the
`-ftest-coverage` and `-fprofile-arcs` flags on the gcc and ld command lines.
It also sets DONT_OPTIMIZE. Note: If you use the `--enable-coverage` configure
flag, you can't turn it off via menuselect so choose one method and stick to
it.

KEEP_FRAME_POINTERS: This option sets `-fno-omit-frame-pointers` on the gcc
command line which can facilitate debugging with 'gdb' and tracing with 'perf'.
Unlike CODE_COVERAGE, this option doesn't depend on optimization being
disabled.  It does however conflict with COMPILE_DOUBLE.