Refactors the SONiC Installer to support greater flexibility in building for a given architecture and bootloader.
#### Why I did it
Currently the SONiC installer assumes that if a platform is ARM based that it uses the `uboot` bootloader and uses the `grub` bootloader otherwise. This is not a correct assumption to make as ARM is not strictly tied to uboot and x86 is not strictly tied to grub.
#### How I did it
To implement this I introduce the following changes:
* Remove the different arch folders from the `installer/` directory
* Merge the generic components of the ARM and x86 installer into `installer/installer.sh`
* Refactor x86 + grub specific functions into `installer/default_platform.conf`
* Modify installer to call `default_platform.conf` file and also call `platform/[platform]/patform.conf` file as well to override as needed
* Update references to the installer in the `build_image.sh` script
* Add `TARGET_BOOTLOADER` variable that is by default `uboot` for ARM devices and `grub` for x86 unless overridden in `platform/[platform]/rules.mk`
* Update bootloader logic in `build_debian.sh` to be based on `TARGET_BOOTLOADER` instead of `TARGET_ARCH` and to reference the grub package in a generic manner
#### How to verify it
This has been tested on a ARM test platform as well as on Mellanox amd64 switches as well to ensure there was no impact.
#### Description for the changelog
[arm] Refactor installer and build to allow arm builds targeted at grub platforms
#### Link to config_db schema for YANG module changes
N/A
