update-kernel: Make path for tmpdir changeable via cli flag

On a Raspberry Pi 3B+ the memory is limited. Updating a kernel might be not possible due to "out of space" situations. This MR adds a flag to mitigate the problem for users facing this issue. This should fix #10470 (closed).

As a workaround I changed the file /sbin/update-kernel directly:

# update-my-kernel
Parallel mksquashfs: Using 4 processors
Creating 4.0 filesystem on /storage/tmp/update-kernel.I6CM4c/boot/modloop-rpi, block size 131072.
[========================================================================================================|] 2062/2062 100%

Exportable Squashfs 4.0 filesystem, xz compressed, data block size 131072
        compressed data, compressed metadata, compressed fragments,
        compressed xattrs, compressed ids
        duplicates are removed
Filesystem size 25936.57 Kbytes (25.33 Mbytes)
        29.00% of uncompressed filesystem size (89441.28 Kbytes)
Inode table size 16554 bytes (16.17 Kbytes)
        24.34% of uncompressed inode table size (68006 bytes)
Directory table size 20128 bytes (19.66 Kbytes)
        43.85% of uncompressed directory table size (45901 bytes)
Number of duplicate files found 13
Number of inodes 2065
Number of files 1711
Number of fragments 347
Number of symbolic links  0
Number of device nodes 0
Number of fifo nodes 0
Number of socket nodes 0
Number of directories 354
Number of ids (unique uids + gids) 1
Number of uids 1
        root (0)
Number of gids 1
        root (0)