firmware: Allocate space for quantizer table in flash

It is important to ensure that the addresses and lengths of the existing
NVM sections (settings and lut) are not changed by this patch.
I have manually verified this as follows:

* Build the firmware before applying this commit
* Run:
    $ objdump -t build/gemini-firmware.elf | grep _nvm
* Then apply the commit and rebuild
* Run objdump again
* Compare the two sets of outputs. The relevant columns are the first
  (symbol value) and last (symbol name)

Author: aquaticsarah

firmware/scripts/samd21g18a.ld

@@ -55,6 +55,20 @@ BOOTLOADER_SIZE = 0x2000;  /* 8kB, 8,192 bytes */
 NVM_SIZE = 0x400;          /* 1kB, 1,024 bytes */
 SRAM_SIZE = 0x8000;        /* 32kB, 32,768 bytes */
 
+/*
+   We reserve a chunk at the end of the flash for "non-volatile memory" (NVM) -
+   which is used by the application to store per-device information.
+   This is divided into three sections:
+
+   * User-selected quantizer table
+   * User settings
+   * DAC calibration lookup table (LUT)
+*/
+QUANTIZER_SIZE = 0x1000;   /* 4kB, 4,096 bytes - TODO reduce once format is settled */
+SETTINGS_SIZE = 0x200;     /* 0.5kB, 512 bytes */
+LUT_SIZE = 0x200;          /* 0.5kB, 512 bytes */
+NVM_SIZE = QUANTIZER_SIZE + SETTINGS_SIZE + LUT_SIZE;
+
 /*
    ARM Cortex-M processors use a descending stack and generally
    require stack space to be set aside in RAM.
@@ -530,27 +544,38 @@ SECTIONS
    to use.
 */
 
+/*
+   Symbols for the quantizer section in the NVM memory block.
+
+   Gemini uses this section to store the user's custom quantization
+   table (if any).
+
+   References:
+   * ../src/gem_quantizer.c
+   TODO: Add a marker to the packed config, and hook up to this symbol
+*/
+_nvm_quantizer_base_address = ORIGIN(nvm);
+_nvm_quantizer_length = QUANTIZER_SIZE;
+
 /*
    Symbols for the settings section in the NVM memory block.
 
-   Gemini uses the first half of the NVM block for user settings.
-   This symbol is used by the settings module to know where to
-   load and save settings.
+   Gemini uses this section to load and save user settings.
 
    References:
    * ../src/gem_settings_load_save.c
 */
-_nvm_settings_base_address = ORIGIN(nvm);
-_nvm_settings_length = LENGTH(nvm) / 2;
+_nvm_settings_base_address = ORIGIN(nvm) + QUANTIZER_SIZE;
+_nvm_settings_length = SETTINGS_SIZE;
 
 /*
    Symbols for the calibration/look-up table in the NVM memory block.
 
-   Gemini uses the other half of the NVM block to store the factory-
-   calibrated look-up table for translating ADC -> frequency/DAC codes.
+   Gemini uses this section to store the factory-calibrated look-up table
+   for translating ADC -> frequency/DAC codes.
 
    References:
    * ../src/gem_ramp_table_load_save.c
 */
-_nvm_lut_base_address = ORIGIN(nvm) + LENGTH(nvm) / 2;
-_nvm_lut_length = LENGTH(nvm) / 2;
+_nvm_lut_base_address = ORIGIN(nvm) + QUANTIZER_SIZE + SETTINGS_SIZE;
+_nvm_lut_length = LUT_SIZE;