📚 [SW makefile] add variables to help target

[stnolting]

This PR adds a help text regarding the makefile's variables to the help target (inspired by #232):

$ make
<<< NEORV32 SW Application Makefile >>>
Make sure to add the bin folder of RISC-V GCC to your PATH variable.

== Targets ==
 help         - show this text
 check        - check toolchain
 info         - show makefile/toolchain configuration
 exe          - compile and generate <neorv32_exe.bin> executable for upload via bootloader
 hex          - compile and generate <neorv32_exe.hex> executable raw file
 image        - compile and generate VHDL IMEM boot image (for application) in local folder
 install      - compile, generate and install VHDL IMEM boot image (for application)
 sim          - in-console simulation using default/simple testbench and GHDL
 all          - exe + hex + install
 elf_info     - show ELF layout info
 clean        - clean up project
 clean_all    - clean up project, core libraries and image generator
 bl_image     - compile and generate VHDL BOOTROM boot image (for bootloader only!) in local folder
 bootloader   - compile, generate and install VHDL BOOTROM boot image (for bootloader only!)

== Variables ==
 USER_FLAGS   - Custom toolchain flags [append only], default ""
 EFFORT       - Optimization level, default "-Os"
 MARCH        - Machine architecture, default "rv32i"
 MABI         - Machine binary interface, default "ilp32"
 APP_INC      - C include folder(s) [append only], default "-I ."
 ASM_INC      - ASM include folder(s) [append only], default "-I ."
 RISCV_PREFIX - Toolchain prefix, default "riscv32-unknown-elf-"
 NEORV32_HOME - NEORV32 home folder, default "../../.."