Improve esp32 flash.sh tool

Enhances the `flash.sh` tool to accept `-o` option to set the flash offset from option arguments,
overriding any environment variable. Also adds the `-l` option to allow flashing `atomvmlib.avm` to
the default `lib.avm` partiton offset of `0x1D0000`. Allows for the use of independently installed
`esptool.py`, either from OS packages, with `pip install esptool`, or from Espressif's
[esptool github repo](https://github.com/espressif/esptool). Note: by default, if the ESP-IDF has
been activated in the users environment, the esp-idf included version of the tool will appear first
in the users environment PATH.

Signed-off-by: Winford <winford@object.stream>

tools/dev/flash.sh

@@ -25,28 +25,35 @@ set -e
 : "${FLASH_SERIAL_PORT:=/dev/ttyUSB0}"
 : "${FLASH_OFFSET:=0x210000}"
 
-if [ -z "${IDF_PATH}" ]; then
-    echo "ERROR!  IDF_PATH must be set to root of IDF-SDK to pick up esptool."
+ESP_TOOL=`which esptool.py`
+if [ -z ${ESP_TOOL} ]; then
+    echo "ERROR!  esptool.py not found! IDF_PATH must be set to root of IDF-SDK to pick up esptool. Or install the standalone tool."
     exit -1
 fi
 
 usage() {
-    echo "Usage: $0 [-p <port>] [-b <baud>] <avm_file>"
+    echo "Usage: $ ${0} [-p <port>] [-b <baud>] [-o <offset>] [-l :: flash given .avm file to lib.avm partition] <avm_file>"
 }
 
 if [[ $# -lt 1 ]]; then
     usage
     exit -1
 fi
 
-while getopts 'p:b:h' arg; do
+while getopts 'p:b:o:lh' arg; do
     case ${arg} in
         p)
             FLASH_SERIAL_PORT=${OPTARG}
             ;;
         b)
             FLASH_BAUD_RATE=${OPTARG}
             ;;
+        o)
+            FLASH_OFFSET=${OPTARG}
+            ;;
+        l)
+            FLASH_OFFSET="0x1D0000"
+            ;;
         h)
             usage
             exit 0