Makefile: remove target 'boot' and fixboot script

To make ROM code load at91bootstrap we need put a the file size in
vector table.
As we use .lds to calcuate the _romsize, and put it in the vector
table of generated binary file.
So we don't need 'boot' target and fixboot script to do this any more.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
config: board: change "Bus Speed" choice appearance's dependence.

Change to depend on the board's decision, instead of the "CPU clock" selection.

Because one CPU clock will export several bus speeds,
but some bus speeds which aren't used on the board make user confused.

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>