build(usr): make usr build script dash compliant

[AndreCostaaa]

Both dash and bash seem to work. Could you please test on your end? @daniel-rossier

> dash build.sh
Platform is virt64
Starting Debug build
Not searching for unused variables given on the command line.
System is unknown to cmake, create:
Platform/SO3_usr to use this system, please post your config file on discourse.cmake.org so it can be added to cmake
Your CMakeCache.txt file was copied to CopyOfCMakeCache.txt. Please post that file on discourse.cmake.org.
-- Configuring done (0.0s)
-- Generating done (0.2s)
-- Build files have been written to: /home/andre/dev/so3/usr/build
[  0%] Built target slv
[ 16%] Built target c
[ 16%] Linking C executable init.elf
[ 16%] Linking C executable sh.elf
[ 17%] Linking C executable more.elf
[ 17%] Linking C executable time.elf
[ 17%] Linking C executable mydev_test.elf
[ 17%] Linking C executable ping.elf
[ 17%] Linking C executable ls.elf
[ 17%] Built target mydev_test.elf
[ 17%] Built target more.elf
[ 17%] Built target ls.elf
[ 17%] Built target time.elf
[ 17%] Built target init.elf
[ 17%] Built target sh.elf
[ 18%] Built target ping.elf
[ 51%] Built target lvgl
[ 55%] Built target lvgl_thorvg
[ 74%] Built target lvgl_examples
[ 99%] Built target lvgl_demos
[ 99%] Linking C executable lvgl_demo.elf
[ 99%] Linking C executable lvgl_perf.elf
[100%] Linking C executable lvgl_benchmark.elf
[100%] Built target lvgl_perf.elf
[100%] Built target lvgl_benchmark.elf
[100%] Built target lvgl_demo.elf

> bash build.sh
Platform is virt64
Starting Debug build
Not searching for unused variables given on the command line.
System is unknown to cmake, create:
Platform/SO3_usr to use this system, please post your config file on discourse.cmake.org so it can be added to cmake
Your CMakeCache.txt file was copied to CopyOfCMakeCache.txt. Please post that file on discourse.cmake.org.
-- Configuring done (0.0s)
-- Generating done (0.2s)
-- Build files have been written to: /home/andre/dev/so3/usr/build
[  0%] Built target slv
[ 16%] Built target c
[ 16%] Linking C executable init.elf
[ 16%] Linking C executable ls.elf
[ 17%] Linking C executable time.elf
[ 17%] Linking C executable ping.elf
[ 17%] Linking C executable sh.elf
[ 17%] Linking C executable more.elf
[ 17%] Linking C executable mydev_test.elf
[ 17%] Built target mydev_test.elf
[ 17%] Built target time.elf
[ 17%] Built target ls.elf
[ 17%] Built target init.elf
[ 17%] Built target more.elf
[ 18%] Built target ping.elf
[ 18%] Built target sh.elf
[ 51%] Built target lvgl
[ 55%] Built target lvgl_thorvg
[ 74%] Built target lvgl_examples
[ 99%] Built target lvgl_demos
[100%] Linking C executable lvgl_benchmark.elf
[100%] Linking C executable lvgl_demo.elf
[100%] Linking C executable lvgl_perf.elf
[100%] Built target lvgl_perf.elf
[100%] Built target lvgl_demo.elf
[100%] Built target lvgl_benchmark.elf