boards: nxp: convert 'BOOT_HEADER_ENABLE' to Kconfig options

1. This commit converts the 'BOOT_HEADER_ENABLE' preprocessor
define to Kconfig options across NXP i.MX RTxxx evaluation boards.
The changes enable flexible boot header configuration through the
Kconfig build system instead of compile-time defines, improving
configurability and maintainability.

2. Now this flag is not used in Zephyr tree, so removed
'zephyr_compile_definitions(BOOT_HEADER_ENABLE=1)' from the
CMakeLists.txt files of the following boards:
- mimxrt595_evk
- mimxrt685_evk
- mimxrt700_evk

Note: for out-of-tree projects that still require this flag,
users can define it in their own CMakeLists.txt or prj.conf files.

Signed-off-by: Zhaoxiang Jin <Zhaoxiang.Jin_1@nxp.com>