Add option -r to env import to allow import of text files with CRLF as line endings

When this option is enabled, CRLF is treated like LF when importing environments
from text files, which means CRs ('\r') in front of LFs ('\n') are just ignored.

Drawback of enabling this option is that (maybe exported) variables which have
a trailing CR in their content will get imported without that CR. But this
drawback is very unlikely and the big advantage of letting Windows user create
a *working* uEnv.txt too is likely more welcome.

Signed-off-by: Alexander Holler <holler@ahsoftware.de>