tools/btdecode.sh: make grep overridable

This script seems to assume GNU grep, which is not commonly
available as "grep" on some platforms.  For examples, it's
more common to have it named "ggrep" on BSD-based systems
including macOS.

Ideally, I suppose we should avoid GNU dependencies like this
in general.  But I'm not motivated enough to rewrite this script
in a portable way today.

cf. https://www.gnu.org/software/grep/manual/grep.html#grep-Programs-1

tools/btdecode.sh

@@ -40,6 +40,8 @@ USAGE="USAGE: ${0} chip|toolchain-addr2line backtrace_file
 If the first argument contains 'addr2line', it will be used as the toolchain's addr2line tool.
 Otherwise, the script will try to identify the toolchain based on the chip name."
 
+GREP=${GREP:-grep}
+
 VALID_CHIPS=(
   "esp32"
   "esp32s2"
@@ -119,8 +121,8 @@ while read -r line; do
   fi
 
   if [[ $line =~ (\[CPU[0-9]+\]\ )?sched_dumpstack: ]]; then
-    task_id=$(echo $line | grep -oP 'backtrace\|\s*\K\d+')
-    addresses=$(echo $line | grep -oP '0x[0-9a-fA-F]+')
+    task_id=$(echo $line | ${GREP} -oP 'backtrace\|\s*\K\d+')
+    addresses=$(echo $line | ${GREP} -oP '0x[0-9a-fA-F]+')
     if $in_dump_tasks_section; then
       if [[ -n "${backtraces_after[$task_id]}" ]]; then
         backtraces_after[$task_id]="${backtraces_after[$task_id]} $addresses"
@@ -155,4 +157,4 @@ if $in_dump_tasks_section; then
     done
     echo ""
   done
-fi
+fi