kernel.bbclass: do_strip: allow recipes to strip the kernel

Allow recipes to specify sections to be stripped from the kernel output
using KERNEL_IMAGE_STRIP_EXTRA_SECTIONS.  For example:

KERNEL_IMAGE_STRIP_EXTRA_SECTIONS = ".comment .unwanted"

The kernel output is stripped in place.

Since the toolchain does not give indication when the specified sections
are absent, we read the sections first and make this report by issuing a
warning to the developer.

The toolchain by default strips the image with the -s option (even
when -s is not specified):
-s --strip-all       Remove all symbol and relocation information

For example, these sections are always removed:
.debug_aranges
.debug_info
.debug_abbrev
.debug_line
.debug_frame
.debug_str
.debug_loc
.debug_ranges
.symtab
.strtab

In addition to these, the sections listed in
KERNEL_IMAGE_STRIP_EXTRA_SECTIONS will also be removed.

Only stripping of vmlinux (elf) is supported at this time.  A warning
will be given if the image type is not vmlinux.

Stripping the image could also be done in the kernel, but that would
only work for linux-yocto based kernels, so it's not the route we
decided to go.

[YOCTO 3515]

Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Michel Thebeau <michel.thebeau@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>