chip-tool: add more flexibility in PAA root folder selection

[bluebin14]

Problem

chip-tool currently can only select a PAA root folder by command line argument. This presents an integration problem in various existing certification tools since a large amount of calls in several scripts would need to be changed. Instead there is a need for more flexibility in specifying PAA root folder which only requires a backwards compatible change to chip-tool.

Proposed Solution

The following to be applied to chip-tool if --paa-trust-store-path is not given:

if ~/paa-root-certs exists use that
else if environment variable PAA_ROOT_CERTS exists use that 

[tcarmelveilleux]

Assigned to @kpschoedel, he may have time within the next few days. Otherwise we will reassign.

I would recommend against ~/paa-root-certs -> this can be achieved by having PAA_ROOT_CERTS environment variable set to ~/paa-root-certs in the shell's profile by default, which would avoid special casing.

[bluebin14]

yes, PAA_ROOT_CERTS should be sufficient