scripts/download.pl: make the download tool configurable

Introduce a new option in the "Advanced configuration options" to
configure a custom download tool.

By declaring a string in "Use custom download tool" an user can force
what command to use to download package. With the string empty the
default tool used is curl, with wget as a fallback if not available.

download.pl supports 3 tools officially aria2c, curl and wget.
If one of the tool is used in this config, download.pl will use the
default args to make use of them.

If the provided string is different than aria2c, curl or wget, the command
is used as is and the download url will be appended at the end of such command.

While at it also tweak the tool selection logic and chose the tool only
once when the script is called and move aria2c specific variables in the
relevant section.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>

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