Check for enough space on /boot volume (check #209)

[mxklb]

Implements part one of #209

• check that enough space is left on the /boot partition, otherwise skip installation

This prevents the install.sh script to brick the OS (leaving it in read-only mode) if the free /boot space on an OS isn't enough for downgrading the kernel. There must be at least 25M free /boot space to perform the driver installation, otherwise the installation is skipped.

[mxklb]

I choose 25M due to the output

Connecting to archive.raspberrypi.org (archive.raspberrypi.org)|93.93.130.214|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 24898072 (24M) [application/x-debian-package]
Saving to: ‘/tmp/raspberrypi-kernel-headers_1.20190925+1-1_armhf.deb’

/tmp/raspberrypi-kernel-headers_1.20 100%[======================================================================>]  23.74M  2.62MB/s    in 9.3s    

2020-02-14 14:14:05 (2.57 MB/s) - ‘/tmp/raspberrypi-kernel-headers_1.20190925+1-1_armhf.deb’ saved [24898072/24898072]

dpkg: warning: downgrading raspberrypi-kernel-headers from 1.20200212-1 to 1.20190925+1-1
(Reading database ... 86007 files and directories currently installed.)
Preparing to unpack .../raspberrypi-kernel-headers_1.20190925+1-1_armhf.deb ...
Unpacking raspberrypi-kernel-headers (1.20190925+1-1) over (1.20200212-1) ...
Setting up raspberrypi-kernel-headers (1.20190925+1-1) ...
run-parts: executing /etc/kernel/header_postinst.d/dkms 4.19.75+
run-parts: executing /etc/kernel/header_postinst.d/dkms 4.19.75-v7+
run-parts: executing /etc/kernel/header_postinst.d/dkms 4.19.75-v7l+
run-parts: executing /etc/kernel/header_postinst.d/dkms 4.19.75-v8+
grep: /boot/kernel.img: No such file or directory
dd: failed to open '/boot/kernel.img': No such file or directory

gzip: stdin: unexpected end of file
 !!! Your kernel version is 
     Not found *** coressponding *** kernel headers with apt-get.
     This may occur if you have ran 'rpi-update'.
 Choose  *** y *** will revert the kernel to version 4.19.75+ then continue.
 Choose  *** N *** will exit without this driver support, by default.
Would you like to proceed? (y/N)

I don't know if this is enough - the right threshold. Actually I'm unable to test how much free /boot space is really needed to perform the kernel downgrade successfully. So I took 25M as a best guess because of this output. This threshold may get changed in future.

But finally it prevents the install.sh script to brick the OS for low /boot space < 25M conditions.