fs_mgr: Fix kernel command line parsing

Remove new line character ('\n') from the kernel command line after
reading from '/proc/cmdline'. This character is not contained in the
original string and is added as a result of reading (according to
kernel source codes [1]):

    ...
    seq_puts(m, saved_command_line);
    seq_putc(m, '\n');
    ...

As a result, this may corrupt the last argument of the string. For
example, if the last argument is 'androidboot.slot_suffix=_a', then the
target partition ('vendor_a\n') will not be found in fstab section of
the device tree.

[1] fs/proc/cmdline.c

Change-Id: I96a853f1f55f27d782afe2ca8c0b006a75368149
Signed-off-by: Ruslan Trofymenko <ruslan.trofymenko@linaro.org>