generate.sh more portable by removing user specific path

[jim-bcom]

Keep hard coded values of previous version for ease of use (e.g. grpc installation dir, default database relative path).

So without parameters, behavior remains the same as before with default values:

 $ ./generate.sh
XPCF_MODULE_ROOT not set, using default
XPCF_MODULE_ROOT=/home/jmhenaff/.remaken/packages/linux-gcc
DATABASE_DIR not set, using default
DATABASE_DIR=../build-SolARFramework-Desktop_Qt_5_15_2_GCC_64bit-Release
File argument is missing : parsing every file listed in database
[simple file parser] [info] parsing file '/home/jmhenaff/work/SolAR/SolAR/core/SolARFramework/src/core/Log.cpp'

Also add a more complete command line, see examples:

1. A help command

$ ./generate.sh --help
Generate gRPC stubs and services for SolAR services interfaces

Usage:
generate.sh [<option>=<value> ]+
Options:
   -x, --xpcf-module-root: path to XPCF modules (default: /home/jmhenaff/.remaken/packages/linux-gcc)
   -d, --database-dir: directory of SolAR framework generated database file (default: ../build-SolARFramework-Desktop_Qt_5_15_2_GCC_64bit-Release)

2. parameters to override default values for XPCF_MODULE_ROOT or path to compilation database (default path contains Qt version, which may not work on all machines)

   $ ./generate.sh -x=XPCF -d=DATABASE
   XPCF_MODULE_ROOT=XPCF
   DATABASE_DIR=DATABASE
   ./generate.sh: line 99: XPCF/xpcf_grpc_gen/1.0.0/bin/x86_64/static/release/xpcf_grpc_gen: No such file or directory

3. XPCF_MODULE_ROOT env var can be used and overriden

   $ XPCF_MODULE_ROOT=EnvVarXPCF ./generate.sh -x=XPCF -d=DATABASE
   XPCF_MODULE_ROOT env var is defined to EnvVarXPCF
   XPCF_MODULE_ROOT is overriden by user
   XPCF_MODULE_ROOT=XPCF
   DATABASE_DIR=DATABASE
   ./generate.sh: line 99: XPCF/xpcf_grpc_gen/1.0.0/bin/x86_64/static/release/xpcf_grpc_gen: No such file or directory