arm: cortex-m: Implement CONFIG_TEXT_SECTION_OFFSET

On other targets, CONFIG_TEXT_SECTION_OFFSET allows the entire image to
be moved in memory to allow space for some type of header.  The Mynewt
project bootloader prepends a small header, and this config needs to be
supported for this to work.

The specific alignment requirements of the vector table are chip
specific, and generally will be a power of two larger than the size of
the vector table.

Change-Id: I631a42ff64fb8ab86bd177659f2eac5208527653
Signed-off-by: David Brown <david.brown@linaro.org>