mkzenboot: Fix double close of bootfd on error

cppcheck reported:
[rockbox/tools/mkzenboot.c:1176]: (error) Used file that is not opened.

Change-Id: Ibbf7ab2910c7f43b547fef25c62e0b4d19ff9551
sb1: fix buggy comparison

cppcheck reported:
[rockbox/utils/imxtools/sbtools/sb1.c:440]: (warning) Comparison of a boolean expression with an integer other than 0 or 1.

Thanks to Thomas Jarosch

Change-Id: I0078232706d4014a1f2acea310a7a0d0edf7788b
Remove useless assignment of 'len'

The variable 'len' is not used after this statement.
Probably a copy'n'paste leftover from the similar
looking block above.

cppcheck reported:
[rockbox/utils/zenutils/source/shared/cenc.cpp:212]: (style) Same expression on both sides of '-'.

Change-Id: Ia8357187ed39d3fab10d97df75a1146c4f733790