x86: Improve debugging experience
x86: Improve debugging experience

This patch appends some gcc options to CFLAGS when building the default
image in order to improve the debugging experience on GDB.

We use the '-ggdb' option which produces debugging information used by
GDB (including GDB extensions) with level 3 which includes preprocessor
macros information. We also use '-Og' which enables optimizations that
do not interfere with debugging. According to gcc manpage, it should be
the optimization level of choice for the standard edit-compile-debug
cycle, offering a reasonable level of optimization while maintaining
fast compilation and a good debugging experience.

Also, this patch removes the '-g' option from the default CFLAGS because
there is no point in using it when BUILD_RELEASE=1.

As expected, the overall ELF image increases (due to -ggdb3 option) while
the .text section is reduced (due to -Og). For the sake of comparison,
below follows the output of 'size'.

Before patch:
$ size -A hello-world.galileo
hello-world.galileo  :
section           size      addr
.text            13766   1048576
.rodata            241   1064960
.eh_frame         5160   1065204
.eh_frame_hdr     1212   1070364
.data             1188   1073152
.bss             12808   1077248
.debug_info      14351         0
.debug_abbrev     6281         0
.debug_aranges     768         0
.debug_line       6443         0
.debug_str        4805         0
.comment            17         0
.note               40         0
.debug_ranges       24         0
Total            67104

After patch:
$ size -A hello-world.galileo
hello-world.galileo  :
section            size      addr
.text             11718   1048576
.rodata             249   1060864
.eh_frame          5496   1061116
.eh_frame_hdr      1204   1066612
.data              1156   1069056
.bss              12808   1073152
.debug_info       16727         0
.debug_abbrev      7254         0
.debug_loc         2083         0
.debug_aranges      768         0
.debug_macro      17273         0
.debug_line       13433         0
.debug_str        42192         0
.comment             17         0
.note                40         0
Total            132418

MohamedSeliem/contiki,MohamedSeliem/contiki,arurke/contiki,arurke/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,arurke/contiki,arurke/contiki,MohamedSeliem/contiki,arurke/contiki,arurke/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,arurke/contiki