utils/getdeveloperlib.py: use relative paths for files

Using absolute paths within getdeveloperlib isn't very sensible, it
makes a lot more sense to handle everything as relative paths from the
top-level Buildroot source directory.

parse_developers() is changed to no longer take the base path as
argument: it is automatically calculated based on the location of
utils/getdeveloperlib.py. Then, the rest of the logic is adjusted to
use relative paths, and prepend them with the base "brpath" when
needed.

This commit allows pkg-stats to report correct developers information
even when executed from an out of tree directory.

Before this patch:

$ ~/buildroot/support/scripts/pkg-stats -p ipmitool --json out.json
$ cat out.json | jq '.packages.ipmitool.developers'
[]

$ cat out.json | jq '.defconfigs.stm32f469_disco'
{
  "name": "stm32f469_disco",
  "path": "configs/stm32f469_disco_defconfig",
  "developers": []
}

After this patch:

$ ~/buildroot/support/scripts/pkg-stats -p ipmitool --json out.json
$ cat out.json | jq '.packages.ipmitool.developers'
[
  "Floris Bos <bos@je-eigen-domein.nl>",
  "Heiko Thiery <heiko.thiery@gmail.com>"
]
$ cat out.json | jq '.defconfigs.stm32f469_disco'
{
  "name": "stm32f469_disco",
  "path": "configs/stm32f469_disco_defconfig",
  "developers": [
    "Christophe Priouzeau <christophe.priouzeau@st.com>"
  ]
}

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Tested-by: Heiko Thiery <heiko.thiery@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
(cherry picked from commit 40bb37bd70)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

support/scripts/pkg-stats

@@ -969,7 +969,7 @@ def __main__():
     print("Build package list ...")
     packages = get_pkglist(args.npackages, package_list)
     print("Getting developers ...")
-    developers = parse_developers(brpath)
+    developers = parse_developers()
     print("Build defconfig list ...")
     defconfigs = get_defconfig_list()
     for d in defconfigs: