cc2538: Sort link input sections by alignment to optimize size

Normally, the linker does not sort files and sections matched by wildcards, so
they are placed in the order in which they are seen during link. If numerous
objects with different alignments are mixed, or if objects with unusually large
alignments are present, this very likely leads to a lot of space being wasted
because of accumulated alignment gaps.

This commit forces input sections to be sorted by alignment (unless this is
overridden by the linker script), which decreases the number and the size of
alignment gaps, thus saving space.

For a typical Contiki project, this change saves nearly 1 kiB, mainly in .bss.

Note that this behavior is only enabled if the SMALL make variable is set to 1,
because this makes more sense for a size optimization.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
cc2538: Move SoC data to a dedicated section to save space

Some SoC data requires huge alignments. E.g., the µDMA channel control table has
to be 1024-byte aligned, which can waste up to 1023 bytes of RAM because of the
alignment gap. Even more space can be wasted because this forces the alignment
of the .bss section to be at least 1024 bytes. Hence, such SoC data should be
carefully placed instead of relying on the linker.

One solution could have been to move the .bss section before the .data section,
but such data could also appear in the .data section. Consequently, the best
solution seems to be to create a dedicated section to place such SoC data
requiring huge alignments, and to put this section at the beginning of the SRAM.

In the case of the µDMA channel control table, the data is still zeroed by
udma_init() (instead of also being zeroed as part of .bss).

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
cc2538: Remove the unused vtable section

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>