scripts: west_commands: runners: fix dt-flash handling in some runners

In minichlink and spi_burn, the script checks if dt_flash is True by
checking if the value is "y". But dt_flash is a boolean.
Fix these checks.

Also, when dt-flash is True, spi_burn calculate the address in a
convoluted way, by substrating CONFIG_FLASH_LOAD_OFFSET to
itself.
Simplify this computation.

Signed-off-by: Miguel Gazquez <miguel.gazquez@bootlin.com>