Add PKG_HASH and PKG_BUILDNUMBER support

[wolfv]

I think we may not have support for using PKG_HASH and PKG_BUILDNUMBER in the build string right now (for this one needs to compute the values first, then use jinja to evaluate the template.

[Krande]

Hey @wolfv, am I correct in understanding that PKG_HASH and PKG_BUILDNUM are not possible to include in a custom build string in a boa recipe? If so are there any workarounds to get the current package-hash into the build string in case we need to finetune build numbers of package variants (while keeping the hash) like this?

I would like to do something like this in a boa recipe

build:
  number: 4
  string: 'aster_h{{ PKG_HASH }}_{{ PKG_BUILDNUM }}'

But unfortunately it results in -aster_h_.tar.bz2 :(

I am using boa==0.15.1 btw.