boards/frdm: remove exporting unset OPENOCD_EXTRA_INIT

[cladmi]

Contribution description

Remove exporting the unset OPENOCD_EXTRA_INIT
The default value in openocd.sh is being empty, so no use for exporting it.

Testing procedure

The first match shows the default value is empty.
And there is no definition of OPENOCD_EXTRA_INIT in the frdm boards (and not in RIOT in general).

git grep -C 1 OPENOCD_EXTRA_INIT

git grep -C 1 OPENOCD_EXTRA_INIT
dist/tools/openocd/openocd.sh-# Extra board initialization commands to pass to OpenOCD
dist/tools/openocd/openocd.sh:: ${OPENOCD_EXTRA_INIT:=}
dist/tools/openocd/openocd.sh-# Debugger interface initialization commands to pass to OpenOCD
--
dist/tools/openocd/openocd.sh-            -f '${OPENOCD_CONFIG}' \
dist/tools/openocd/openocd.sh:            ${OPENOCD_EXTRA_INIT} \
dist/tools/openocd/openocd.sh-            ${OPENOCD_EXTRA_RESET_INIT} \
--
dist/tools/openocd/openocd.sh-            -f '${OPENOCD_CONFIG}' \
dist/tools/openocd/openocd.sh:            ${OPENOCD_EXTRA_INIT} \
dist/tools/openocd/openocd.sh-            -c 'tcl_port ${TCL_PORT}' \
--
dist/tools/openocd/openocd.sh-            -f '${OPENOCD_CONFIG}' \
dist/tools/openocd/openocd.sh:            ${OPENOCD_EXTRA_INIT} \
dist/tools/openocd/openocd.sh-            -c 'tcl_port ${TCL_PORT}' \
--
dist/tools/openocd/openocd.sh-            -f '${OPENOCD_CONFIG}' \
dist/tools/openocd/openocd.sh:            ${OPENOCD_EXTRA_INIT} \
dist/tools/openocd/openocd.sh-            ${OPENOCD_EXTRA_RESET_INIT} \

Issues/PRs references

Found after cleaning #12151

[fjmolinas]

Changes make sense, I verified there is no usage for OPENOCD_EXTRA_INIT. ACK

[cladmi]

Thank you for the review.

[cladmi]

I first was thinking about completely removing OPENOCD_EXTRA_INIT, but I do not know if there is not a use case. So better keep it for the moment.

[fjmolinas]

I first was thinking about completely removing OPENOCD_EXTRA_INIT, but I do not know if there is not a use case. So better keep it for the moment.

I Think its useful even if never assigned a variable. You can easily test command variation by assigning a value to OPENOCD_EXTRA_INIT when calling make.

[cladmi]

That is a good use case.