Fix insertkeys.py to resolve keys.conf path entries in a portable way

Currently a path to a key in keys.conf must be fully qualified or have
the -d option appended. This fix will allow paths to have environment
variables that will be expanded. This will give portability to the
entries. For example the following entry will now be resolved correctly:
[@NET_APPS]
ALL : $ANDROID_BUILD_TOP/device/demo_vendor/demo_dev/security/net_apps.x509.pem

Change-Id: If4f169d9ed4f37b6ebd062508de058f3baeafead
Signed-off-by: Richard Haines <richard_c_haines@btinternet.com>