Revert "Don't force kernel downgrade"

This reverts commit 30276c80bc.

install.sh

@@ -1,5 +1,7 @@
 #!/bin/bash
 
+FORCE_KERNEL="1.20190925+1-1"
+
 if [[ $EUID -ne 0 ]]; then
    echo "This script must be run as root (use sudo)" 1>&2
    exit 1
@@ -100,15 +102,46 @@ function check_kernel_headers() {
   apt-get -y --reinstall install raspberrypi-kernel
 }
 
+function download_install_debpkg() {
+  local prefix name r
+  prefix=$1
+  name=$2
+
+  for (( i = 0; i < 3; i++ )); do
+    wget $prefix$name -O /tmp/$name && break
+  done
+  dpkg -i /tmp/$name; r=$?
+  rm -f /tmp/$name
+  return $r
+}
+
+function install_kernel() {
+  local _url _prefix
+
+  # Instead of retriving the lastest kernel & headers
+  [ "X$FORCE_KERNEL" == "X" ] && {
+    # Raspbian kernel packages
+    apt-get -y --force-yes install raspberrypi-kernel-headers raspberrypi-kernel
+    # Ubuntu kernel packages
+    apt-get -y install linux-raspi linux-headers-raspi linux-image-raspi
+  } || {
+    # We would like to a fixed version
+    KERN_NAME=raspberrypi-kernel_${FORCE_KERNEL}_armhf.deb
+    HDR_NAME=raspberrypi-kernel-headers_${FORCE_KERNEL}_armhf.deb
+    _url=$(apt-get download --print-uris raspberrypi-kernel | sed -nre "s/'([^']+)'.*$/\1/g;p")
+    _prefix=$(echo $_url | sed -nre 's/^(.*)raspberrypi-kernel_.*$/\1/g;p')
+
+    download_install_debpkg "$_prefix" "$KERN_NAME"
+    download_install_debpkg "$_prefix" "$HDR_NAME"
+  }
+}
+
 # update and install required packages
 which apt &>/dev/null
 if [[ $? -eq 0 ]]; then
   apt update -y
-  # Raspbian kernel packages
-  apt-get -y install raspberrypi-kernel-headers raspberrypi-kernel 
-  # Ubuntu kernel packages
-  apt-get -y install linux-raspi linux-headers-raspi linux-image-raspi
   apt-get -y install dkms git i2c-tools libasound2-plugins
+  install_kernel
   # rpi-update checker
   check_kernel_headers
 fi

seeed-voicecard

@@ -122,7 +122,8 @@ if [ "$overlay" ]; then
     rm /etc/asound.conf
     rm /var/lib/alsa/asound.state
 
-    kernel_ver=$(uname -r) # get_kernel_version)
+: <<\EOF
+    kernel_ver=$(get_kernel_version)
     # echo kernel_ver=$kernel_ver
 
     # TODO: dynamic dtoverlay Bug of v4.19.x
@@ -137,6 +138,7 @@ if [ "$overlay" ]; then
             fi
         done
     fi
+EOF
     #make sure the driver loads correctly
     dtoverlay -d $OVERLAYS $overlay || true