mtd-utils: common.h: fix prompt function

mtd-utils: common.h: fix prompt function

The prompt() function is intended to query a yes/no reply from a command
line user by reading in an entire line of text using getline() and checking
the first character. If the line is empty, a default value is returned.

First of all, this patch replaces the usage of getline() with fgets() to
avoid compilation problems on some smaller C libraries, like klibc, that
do not have a getline() implementation. Since we now have a static line
length, this may break some build setups that input lengthy giberish
instead of 'y' or 'n'.

Second, this patch fixes a more severe bug in prompt(), replacing a 'while'
keyword with the 'if' that was most likely intended. In the old version, if
getline() reported an error, it would print an error message inside a while
loop, immediately followed by a break and then march on and process the
erroneous input instead of using the default value as printed to stdout.

Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>