Its possible to rename the executable we build via the Kconfig symbol
CONFIG_KERNEL_BIN_NAME. So we really should use ${KERNEL_ELF_NAME},
${KERNEL_BIN_NAME} and ${KERNEL_HEX_NAME} variables instead of hardcoded
zephyr.elf, zephyr.bin, and zephyr.elf.
This fixes an build issue with
tests/misc/test_build/buildsystem.kconfig.utf8_in_values on
up_squared_adsp and lpcxpresso11u68 platforms.
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>