remove IAR toolchain support from make build system

IAR is only supported with CMake. Remove all IAR-specific references
from the Make build system including toolchain files, SRC_S_IAR,
LD_FILE_IAR variables, and IAR toolchain detection.

Also fix GET_SECTOR_COUNT truncation in msc_file_explorer and
broken formatting in stm32f7 board_uart_write.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>