Simplify --uboot-init handling
Get rid of trailing newlines in --uboot-init commands

They break our "wait for prompt" logic.
Set $builddir to invocation dir if it is not specified by other means

This fixes undefined variable in --tftp handling when novaboot it run as

  novabot ... -- /dev/null
Initialize some variables even if /dev/null is passed as novaboot script
Improve detection of U-Boot prompt after tftpboot command

If the uboot prompt contains '#' character, it might not be detected
properly.
Add support for $NB_MYIP in --uboot-init