ti: Quote board_init in ti_common.env

ti: Quote board_init in ti_common.env

bootcmd_ti_mmc skips a per-board init hook with:

    if test -n ${board_init}; then run board_init; fi;

The default case is "no board override", i.e. board_init unset. The
expression then expands to 'test -n' with no operand and relies on a
U-Boot 'test' quirk that treats a missing operand as false to skip the
run.

Quote the variable so an unset board_init expands to 'test -n ""' and
the emptiness check is explicit.

Fixes: 8b0619579b22 ("cmd: test: fix handling of single-argument form of test")
Signed-off-by: Simon Glass <sjg@chromium.org>