boards/Kconfig: $BOARD_REVISION -> $(BOARD_REVISION)

Add parentheses missing around $BOARD_REVISION.

The syntax without parentheses has been deprecated for at least 5 years:
https://github.com/ulfalizer/Kconfiglib/commit/374f48873424f9



That same documentation states "Using the old syntax with an undefined
environment variable keeps the string as is." This what actually happens
on Windows where `build/zephyr/.config` looks like this:

  CONFIG_BOARD_REVISION="$BOARD_REVISION"

I found this because the behavior differs on Linux where the same,
"old" syntax produces this instead:

  CONFIG_BOARD_REVISION=""

This could be because environment variables work differently?
(BOARD_REVISION is passed from kconfig.cmake to kconfiglib.py thanks to
cmake -E env)

Let's not try to debug this and just drop the deprecated syntax. It
aligns both Windows and Linux on the same, expected, empty string
behavior.

Note these are the only two `def_string` found across all Kconfig files
right now.

Fixes commit c11b7852 ("Kconfig: add CONFIG_BOARD_REVISION")
Fixes commit e2ff2a88 ("sysbuild: include HWMv2 Kconfig in sysbuild")

Signed-off-by: Marc Herbert <marc.herbert@intel.com>