llext: Add shell command for loading LLEXT from a filesystem

This patch extends LLEXT shell commands with support for loading
LLEXT from a filesystem. Use of the command requires absolute path
to the llext file.

Example use:
`llext load_llext hello_world /lfs/hello_world.llext`

Signed-off-by: Adam Wojasinski <awojasinski@baylibre.com>