Now that every bootloader build has a board info blob,
make all fw and bl images use it.
The following MACROS are removed:
BOARD_TYPE, BOARD_REVISION, BOOTLOADER_VERSION,
START_OF_USER_CODE, HW_TYPE
These values are now ONLY available from the bootloader
flash via the pios_board_info_blob symbol. These values
must not be #defined or otherwise hard-coded into the
firmware in any way. The bootloader flash is the only
valid source for this information.
NOTE: To ensure that we have an upgrade path from an
old bootloader (without board_info_blob) to a
new bootloader (with board_info_blob), it is
essential that the bu_* targets do not depend
on (or validate) the board_info_blob being present
in the bootloader flash.
