flit_core: add variable to control where pyproject.toml is located

flit_core: add variable to control where pyproject.toml is located

The PEP517[1] pyproject.toml doesn't have to be located at the top-level
of the source tree (for example, in recipes with multiple components), so
add a variable PEP517_SOURCE_PATH to set where the file is.

In the future there should be a pep517.bbclass to centralise the common
logic, but for now this can be defined in each of the relevant classes.

[1] https://www.python.org/dev/peps/pep-0517/

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>