flash/nor/bluenrg-x: clarify target algo stack usage

flash/nor/bluenrg-x: clarify target algo stack usage

While on it rename misleading write_algorithm_sp to write_algorithm_stack
and change messages referring 'stack pointer' instead of stack.

No functional change.

Change-Id: Ibb9897d3f01734812ed0f8bc8cd43b935a573f8a
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: https://review.openocd.org/c/openocd/+/7132
Tested-by: jenkins
Reviewed-by: Salvatore Giorgio Pecorino <salvatore-giorgio.pecorino@st.com>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>