Add a new Kconfig symbol that explicitly controls whether the vector
table should be placed in RAM. This eliminates the side effect of
`IS_BOOTLOADER` controlling vector table location. Making the condition
a positive assertion also allows the config to be used in CMakeLists
conditions (`zephyr_linker_sources_ifdef()`, etc).
Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>